Vblank irqs don't get disabled during suspend or driver
unload, which causes irq delivery after "suspend" or
driver unload, at least until the gpu is powered off.
This could race with drm_vblank_cleanup() in the case
of nouveau and cause a use-after-free bug if the driver
is unloaded.

More annoyingly during everyday use, at least on nv50
display engine (likely also others), vblank irqs are
off after a resume from suspend, but the drm doesn't
know this, so all vblank related functionality is dead
after a resume. E.g., all windowed OpenGL clients will
hang at swapbuffers time, as well as many fullscreen
clients in many cases. This makes suspend/resume useless
if one wants to use any OpenGL apps after the resume.

In Linux 3.16, drm_vblank_on() was added, complementing
the older drm_vblank_off()  to solve these problems
elegantly, so use those calls in nouveaus suspend/resume
code.

For kernels 3.8 - 3.15, we need to cherry-pick the
drm_vblank_on() patch to support this patch.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: <stable@vger.kernel.org> #v3.16
Cc: <stable@vger.kernel.org> #v3.8+: f2752282: drm: Add drm_vblank_on()
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>