drm/nouveau: Forbid runtime pm on driver unload

drm/nouveau: Forbid runtime pm on driver unload

The PCI core calls pm_runtime_forbid() on device probe in pci_pm_init(),
making this the default state when nouveau is loaded. nouveau_drm_load()
therefore calls pm_runtime_allow(), but there's no pm_runtime_forbid()
in nouveau_drm_unload() to balance it. Add it so that we leave the
device in the same state that we found it.

This isn't a bug, it's just good housekeeping. When nouveau is first
loaded with runpm=1, then unloaded and loaded again with runpm=0,
pm_runtime_forbid() will be called from nouveau_pmops_runtime_idle() or
nouveau_pmops_runtime_suspend(), so the behaviour is correct. The nvidia
blob doesn't use runtime pm, but if it ever does, this commit avoids
that it has to clean up behind nouveau.

Cc: Ben Skeggs <bskeggs@redhat.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/92cf96445088217a4d7d7081b90140f2d6f047da.1465392124.git.lukas@wunner.de