drm/radeon/kms/cayman/blit: specify CP_COHER_CNTL2 with surface_sync

CP_COHER_CNTL2 has to be programmed manually when submitting packets
to the ring directly rather than programmed via an IB.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>