drm/vc4: Add support for DRM_FORMAT_RGB888 and DRM_FORMAT_BGR888

Filling out the list of supported formats based on those the
hardware can support.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/b551205d1c33fa49eef2c33ed2d60c5339b2f299.1510841336.git.dave.stevenson@raspberrypi.org