drm/radeon/kms: demystify evergreen blit code

some bits in 3D registers used by blit functions look like
magic and this is hard to follow; change them to a little bit
more meaningful pre-defined constants

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