drm/amdgpu: Add a missing lock for drm_mm_takedown

Inside amdgpu_gtt_mgr_fini add a missing lock to maintain
locking balance

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>