drm/vmwgfx: Make sure backing mobs are cleared when allocated. Update driver date.

Backing mob contents is propagated to user-space, so make sure backing
mobs are cleared when allocated. This also accidently fix rendering errors
with celestia when emulating legacy mode.

Also update driver date.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>