drm/radeon/kms: fix the colorbuffer CS checker for r300-r500

drm/radeon/kms: fix the colorbuffer CS checker for r300-r500

This commit fixes bogus CS rejection if it contains a sequence
of the following operations:

- Set the color buffer 0. track->cb[i].robj becomes non-NULL.
- Render.
- Set a larger zbuffer than the previously-set color buffer.
- Set a larger scissor area as well.
- Set the color channel mask to 0 to do depth-only rendering.
- Render. --> rejected, because track->cb[i].robj remained non-NULL,
  therefore the conditional checking for the color channel mask and
  friends is not performed, and the larger scissor area causes
  the rejection.

This fixes bugs:
- https://bugs.freedesktop.org/show_bug.cgi?id=29762
- https://bugs.freedesktop.org/show_bug.cgi?id=28869
And maybe some others which seem to look the same.

If possible, this commit should go to stable as well.

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>