drm/vmwgfx: Refuse DMA operation when SEV encryption is active

TTM doesn't yet fully support mapping of DMA memory when SEV is active,
so in that case, refuse DMA operation. For guest-backed object operation
this means 3D acceleration will be disabled. For host-backed, VRAM will be
used for data transfer between the guest and the device.

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