drm/amdgpu: tear down ttm range manager for doorbell in amdgpu_ttm_fini()

Tear down ttm range manager for doorbell in function amdgpu_ttm_fini(),
to avoid memory leakage.

Fixes: 792b84fb ("drm/amdgpu: initialize ttm for doorbells")
Signed-off-by: Jiang Liu <gerry@linux.alibaba.com>
Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>