radeon: Don't unregister fictitious memory range if not registered

This fixes a panic if GPU initialization fails before the fictitious
memory range is registered. In this case, the shutdown code was
unregistering the range and triggered the panic.
drm: Call "gem_close_object" driver callback from drm_gem_object_release_handle()

This seems to fix leakage of "bo_va" for Cayman and following card
generations.