drm/radeon/kms: make r600-NI blit suspend code common

r600-NI shared the same blit suspend code.  Clean it up
and make it a shared function.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>