drm/vmwgfx/gmrid: don't provide pointless ttm debug callback

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-11-airlied@gmail.com