vmwgfx: Implement GMR2

Guest Memory Regions 2 is a way to bind pages to the GPU, but using
the FIFO instead of an io-submitted descriptor chain.

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