minigbm: rockchip: enable NV12 for Android

minigbm: rockchip: enable NV12 for Android

For NV12 buffers to work on Android, we need to use the same
logic that we use in gralloc_drm_rockchip.  Otherwise, video
playback doesn't work in the container.

BUG=chromium:616275
TEST=Play Youtube app.  Also ran plane_test (which defaults to NV12).
CQ-DEPEND=CL:392547

Change-Id: I8dbd989f2638c99200866f8191450c71114bdbe5
Reviewed-on: https://chromium-review.googlesource.com/392548
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>