drm/nv50: add a memory barrier to pushbuf submission

- This is useful for vram pushbuffers that are write combined.
- pre-nv50 has one too (in WRITE_PUT).

Signed-off-by: Maarten Maathuis <madman2003@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>