drm/radeon: allow CMASK and FMASK in the CS checker on r600-r700

MSAA is impossible without them.

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>