drm/amdgpu: finalizing mem_partitions at the end of GMC v9 sw_fini

The valid num_mem_partitions is required during ttm pool fini,
thus move the cleanup at the end of the function.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>