vmwgfx: Require HWV8 for 3d support

On lower versions, the way we mix 2D and 3D may be too slow.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>