drm/radeon: avoid potential null pointer access

Leverage the same logic from amdgpu_ttm_tt_unpin_userptr.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>