[Intel-gfx] [RFC] Reduce idle vblank wakeups

[Matthew Garrett]

The drm core currently waits 5 seconds from userspace dropping a request
for vblanks to vblanks actually being disabled. This appears to be a
workaround for broken hardware, but results in a mostly idle desktop
generating a huge number of wakeups that are entirely unnecessary but which
consume measurable amounts of power. This patchset makes the vblank timeout
per-device rather than global, making it easy for drivers to override the
behaviour without compromising other graphics hardware in the system. It
then removes the delay on Intel hardware. I've tested this successfully on
Sandybridge without any evidence of spurious or missing irqs, but I don't
know how well it works on older hardware. Feedback not only welcome, but
positively desired.