drm/radeon: skip colorbuffer checking if COLOR_INFO.FORMAT is set to INVALID

This fixes a bug which was causing rejections of valid GPU commands
from userspace.

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>