drm/radeon: fix userptr BO unpin bug v3

Fixing a memory leak with userptrs.

v2: clean up the loop, use an iterator instead
v3: remove unused variable

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