drm/radeon/kms: allow rendering while no colorbuffer is set on r300

Because hardware cannot disable all colorbuffers directly to do depth-only
rendering, a user should:
- disable reading from a colorbuffer in blending
- disable fastfill
- set the color channel mask to 0 to prevent writing to a colorbuffer

Signed-off-by: Dave Airlie <airlied@redhat.com>