amdgpu_driver_load_kms() calls pm_runtime_put_autosuspend() if
amdgpu_device_is_px(dev), but amdgpu_driver_unload_kms() calls
pm_runtime_get_sync() unconditionally. We therefore leak a runtime pm
ref whenever amdgpu is unloaded on a non-PX machine or if runpm=0. The
GPU will subsequently never runtime suspend after loading amdgpu again.
Fix by taking the runtime pm ref under the same condition that it was
released on driver load.