drm/radeon/kms: more atom parser fixes (v2)

shr/shl ops need the full dst rather than the pre-masked
version.  Fixes fdo bug 27478 and kernel bug 15738.

v2: remove some unsed vars, add comments

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