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Make all the LOGs that follow DRM calls PLOGs since we're interested in
the error message.

Also add a few more LOGs during device probing just in case we end up in
odd states where the DRM device is controlled by another process and
we're trying to take ownership.

BUG=none

Review URL: https://codereview.chromium.org/991963002

Cr-Commit-Position: refs/heads/master@{#319716}