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

radeon_driver_load_kms() calls pm_runtime_put_autosuspend() if
radeon_is_px(dev), but radeon_driver_unload_kms() calls
pm_runtime_get_sync() unconditionally. We therefore leak a runtime pm
ref whenever radeon is unloaded on a non-PX machine or if runpm=0. The
GPU will subsequently never runtime suspend after loading radeon again.

Fix by taking the runtime pm ref under the same condition that it was
released on driver load.

Fixes: 10ebc0bc ("drm/radeon: add runtime PM support (v2)")
Cc: Dave Airlie <airlied@redhat.com>
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/aaf71106c042126817aeca8b8e54ed468ab61ef7.1465392124.git.lukas@wunner.de