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The page flip code mixes two error handling styles and in many cases
returns the same status (or in some cases, inconsistent statuses) through
the return value and also via the SwapCompletionCallback.

The page flip code also supports two modes: real page flips and test page
flips, which are used to validate configurations before use. Real page
flips crash the GPU process if the return value indicates failure, but
obviously this is not expected to occur so in practice those always
return success, and pass status information through the
SwapCompletionCallback. Test page flips are the opposite: they must
return their status in the return value, and pass a dummy completion
callback which ignores its arguments.

There are actually no asynchronous failure cases, despite a fair amount
of code trying to account for that possibility such as the coalescing of
SwapResults in PageFlipRequest. We can take advantage of this to avoid
passing the SwapCompletionCallback into most functions, and simply use
the return value as the only status return. The only complication is that
successful completion is always asynchronous, and may even require
tracking multiple underlying page flip events, as in hardware mirroring
scenarios.

This patch handles that issue by registering PageFlipCallbacks for all of
the individual page flip events as usual, but only passing ownership of
the SwapCompletionCallback into the PageFlipRequest at the last minute,
after all of the individual page flips have been submitted succesfully.

After hiding the SwapCompletionCallback from most of the DRM internals,
test page flips no longer need to construct a dummy
SwapCompletionCallback. They also don't need to construct any
PageFlipCallbacks, since they don't generate any page flip events - this
simplifies that logic as well by moving construction of each
PageFlipCallback to immediately after a successful page flip request.
Moving creation of those callbacks later also removes any requirement to
make manual calls to PageFlipRequest::Signal to fixup failure cases.

Since failures of real page flips triggered by SchedulePageFlip() simply
crash the process, there is no need for a return value. This patch moves
the CHECK() which cause these crashes inside of SchedulePageFlip(), and
removes the return value.

As a result, TestPageFlip() and all of the internal methods called via
ActualSchedulePageFlip() use return values exclusively for status
information, and never see the SwapCompletionCallback.
SchedulePageFlip(), on the other hand, receives the SwapCompletionCallack
and uses it exclusively for its status, and returns void.

Bug: 851997
Test: unit tests, various display configurations on cros

Change-Id: I87408275b0958281ef79a6b5086a38b2c3c08ee1
Reviewed-on: https://chromium-review.googlesource.com/1104859
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569973}