drm/radeon: add extra check in radeon_ttm_tt_unpin_userptr

We somehow try to free the SG table twice.

Bugs: https://bugs.freedesktop.org/show_bug.cgi?id=89734



Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>