drm/radeon/kms: clean up some low-hanging magic numbers

Switch some magic numbers to their proper defines.
The register header madness needs to be cleaned up
at some point.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/radeon/r100.c
Simple merge

diff --cc drivers/gpu/drm/radeon/r300.c
Simple merge

diff --cc drivers/gpu/drm/radeon/r420.c
Simple merge