drm/amdgpu: fix userptr BO unpin bug (v2)

sg could point to array of contigiouse page*, only free page could lead
to memory leak.

v2: use iterator

Signed-off-by: monk.liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>