minigbm: virtio: restrict formats supported without 3D

minigbm: virtio: restrict formats supported without 3D

Upstream virtio drm commit "drm/virtio: fix DRM_FORMAT_* handling" severely
restricts supported formats and strictly enforces 32bpp dumb buffers.
Therefore now we only support XRGB8888 for scanout, ARGB8888 for cursor,
ARGB8888 for rendering. Also, we pretend all buffers are 32bpp when allocating
as dumb buffers.

BUG=none
TEST=tast run 127.0.0.1:9222 webrtc.RTCPeerConnection.vp8

Change-Id: I6225a9cb3c49f0850c6d94b2d12efaf9a33b149e
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/1849773
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>