drm/vmwgfx: Fix a surface reference corner-case in legacy emulation mode

If running on a gb-object capable device with a non-gb capable surface
exporter (X server) and a gb capable surface referencing client (GL driver),
the referencing client expects to find a shareable backing buffer attached to
the surface at reference time. This may not be the case if the surface has
not yet been validated. This would cause the surface reference IOCTL to
return an error.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>