drm/ttm: use gem reservation object

Drop ttm_resv from ttm_buffer_object, use the gem reservation object
(base._resv) instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190805140119.7337-8-kraxel@redhat.com