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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. However we don't know in advance
how many different frames the encoder will use, only that it is probably
larger than the default 2 we are currently allocating. If we undershoot
then we will spend a lot of time in-kernel unmapping/remapping DMABUFs.

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
BUG=b:167412992
TEST=video.EncodeAccel.h264_360p_i420 passes on Kukui.

Change-Id: Ia24a6bd8574f5d3894237ca0e563ea4cbff3a631
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415911
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Fritz Koenig <frkoenig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813545}