drm/amdkfd: kfree the wrong pointer

Originally, it kfrees the wrong pointer for mem_obj.
It would cause memory leak under stress test.

Signed-off-by: Jack Zhang <Jack.Zhang1@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>