drm, ttm Fix uninitialized warning

Fix uninitialized warning.

drivers/gpu/drm/ttm/ttm_object.c: In function ‘ttm_base_object_lookup’:
drivers/gpu/drm/ttm/ttm_object.c:213:10: error: ‘base’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  kref_put(&base->refcount, ttm_release_base);
          ^
drivers/gpu/drm/ttm/ttm_object.c:221:26: note: ‘base’ was declared here
  struct ttm_base_object *base;

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>