vmwgfx: Drop 3D Legacy Display Unit support

Since 3D requires HWv8 and screen objects is always available on those
hosts we only need the screen objects path for surfaces.

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