vgem: support DRM_FORMAT_YVU420_ANDROID

vgem: support DRM_FORMAT_YVU420_ANDROID

drv_dumb_bo_create function only allocates single-plane formats at the
moment, even though the vgem driver has a resolve format hook that
leads to DRM_FORMAT_YVU420_ANDROID. Let's allocate a single buffer
that's big enough to support DRM_FORMAT_YVU420_ANDROID, as drm_gralloc
does. Note videos don't work with Android emulation, but they never
did with drm_gralloc and that's a separate problem.

BUG=chromium:616275
TEST=no regressions with gbmtest when running qemu + trybots

Change-Id: I9b081dbe553fe1159feaa7eed104ac340c59e41b
Reviewed-on: https://chromium-review.googlesource.com/456404
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>