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If we are operating in DMABUF mode, then we will try to keep the same
frame assigned to the same V4L2 buffer. The number of buffers requested
by the client is indicative of the real load, but there is no hard
guarantee that we will not receive more frames than that, thus resulting
in sub-optimal affinity tracking.

Try to prevent this by allocating as many buffers as allowed by the V4L2
API (32) when operating in DMABUF mode. Since these buffers won't have
backing memory unless we attach a DMABUF to them, they are virtually
free if unused.

BUG=b:159688625
TEST=video.DecodeAccel.h264 passes on Kukui.

Change-Id: Ibc9ea18835acd4b8d3bc6ffc7393bf0d5d97d04d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415908
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Auto-Submit: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Fritz Koenig <frkoenig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813110}