drm/amdgpu: Don't leak runtime pm ref on driver unload

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.

Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/86364eeb5711323608930c4f0f69046792ff6d3c.1465392124.git.lukas@wunner.de