drm/vmwgfx: Hook up MOBs to TTM as a separate memory type

To bind a buffer object as a MOB, just validate it as a MOB
memory type. We are reusing the GMRID manager, although we create a new
instance of it to manage MOB ids and tomake sure we don't exceed
the maximum amount of MOB pages.

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