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

commit 970a5ee41c72df46e3b0f307528c7d8ef7734a2e upstream.

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+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>