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

nouveau_drm_load() calls pm_runtime_put() if nouveau_runtime_pm != 0,
but nouveau_drm_unload() calls pm_runtime_get_sync() unconditionally.
We therefore leak a runtime pm ref whenever nouveau is loaded with
runpm=0 and then unloaded. The GPU will subsequently never runtime
suspend even if nouveau is loaded again with runpm=1.

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

Fixes: 5addcf0a ("nouveau: add runtime PM support (v0.9)")
Cc: Dave Airlie <airlied@redhat.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Reported-by: Karol Herbst <karolherbst@gmail.com>
Tested-by: Karol Herbst <karolherbst@gmail.com>
Tested-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1544b82007037601fbc510b1a50edc56c529e75f.1465392124.git.lukas@wunner.de