drm/radeon/kms/r600: reduce gpu cache flushing

Only flush the gpu caches before we submit a fence.
This leads to a small performance boost when we take
the extra gpu cache flushes out of the ddx and mesa.
Once this patch is in and the drm version is bumped,
we can remove the flushes from the ddx and drm.

Also, remove the extra cache flushes from the blit
routine.

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