drm/nouveau/instmem/gk20a: add write barrier when releasing DMA object

When using the DMA-API for instmem, we may obtain a write-combined
mapping. For such cases, add a write barrier in
gk20a_instobj_release_dma() to make sure that all writes have reached
memory at this time.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>