drm/ttm: set both resv and base.resv pointers

Initialize both ttm_buffer_object->resv and ttm_buffer_object->base.resv
pointers.  This allows to move users from the former to the latter.  When
all users are moved we can drop ttm_buffer_object->resv.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190805140119.7337-10-kraxel@redhat.com