drm/ttm: Replace ttm_bo_unref() with ttm_bo_put()

A call to ttm_bo_unref() clears the supplied pointer to NULL, while
ttm_bo_put() does not. None of the converted call sites requires the
pointer to become NULL, so the respective assign operations has been
left out from the patch.

Signed-off-by: Thomas Zimmermann <contact@tzimmermann.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>