drm/ttm: drop ttm_buffer_object->resv

All users moved to ttm_buffer_object->base.resv

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190805140119.7337-18-kraxel@redhat.com