mediatek: Change android flexible format to allocate YVU420 buffer

mediatek: Change android flexible format to allocate YVU420 buffer

crrev.com/c/1716864 made gralloc allocae NV12 buffer with the android flexible
format, DRM_FORMAT_FLEX_YCbCr_420_888. However, the change broke many CTS tests.
This partially reverts the commit. The allocated buffer fromat becomes YVU420.

BUG=chromium:987185
BUG=b:139714614
Cq-Depend: chromium:1767680
Change-Id: I9fcde88cff79fac2655fb627418b371b314077c1
Reviewed-on: https://chromium-review.googlesource.com/1767450
Tested-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Ready: Hirokazu Honda <hiroh@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>