drm/radeon/kms/atom: clean up op_mask handler

Readability cleanup and fix debugging output, no
functional change.
Reported-by: Frank Huang <FrankR.Huang@amd.com>

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