drm/nouveau/kms/nv50-: also flush fb writes when rewinding push buffer

Should hopefully fix a regression some people have been seeing since EVO
push buffers were moved to VRAM by default on Pascal GPUs.

Fixes: d00ddd9d ("drm/nouveau/kms/nv50-: allocate push buffers in vidmem on pascal")
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: <stable@vger.kernel.org> # 4.19+