virgl: add texture_barrier stub

virgl: add texture_barrier stub

In gallium, supporting FBFETCH means supporting non-coherent fetches, but
in virglrenderer, due to technical reasons this is backed by coherent
fetches instead. This means we don't need to do anything for the barriers.

However, if we don't have a texture_barrier implementation, we get crashes
because the non-coherent extensions is exposed.

So, let's leave this as a NOP for now.

[airlied: I've got a more complete impl of this somewhere, once we
land the host side].
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>