drm/vmwgfx: Fix the refuse_dma mode when using guest-backed objects

When we refuse DMA from system pages for whatever reason, we don't
handle that correctly when guest-backed objects was enabled.
Since guest-backed objects by definition require DMA to and from
system pages, disable all functionality that relies on them.
That basically amounts to 3D acceleration and screen targets.

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