drm/virtio: Add memory barriers for capset cache.

drm/virtio: Add memory barriers for capset cache.

[ Upstream commit 9ff3a5c88e1f1ab17a31402b96d45abe14aab9d7 ]

After data is copied to the cache entry, atomic_set is used indicate
that the data is the entry is valid without appropriate memory barriers.
Similarly the read side was missing the corresponding memory barriers.

Signed-off-by: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190610211810.253227-5-davidriley@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>