drm/radeon/kms: demystify r600 blit code

some 3d register bits look like magic in r600 blit functions
use predefined constants to make it more intuitive what they are

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>