drm/radeon: use the dummy page for GART if needed

Imported BOs don't have a pagelist any more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: 0575ff3d ("drm/radeon: stop using pages with drm_prime_sg_to_page_addr_arrays v2")
CC: stable@vger.kernel.org # 5.12