drm/i915: add i915.pc8_timeout function

drm/i915: add i915.pc8_timeout function

We currently only enter PC8+ after all its required conditions are
met, there's no rendering, and we stay like that for at least 5
seconds.

I chose "5 seconds" because this value is conservative and won't make
us enter/leave PC8+ thousands of times after the screen is off: some
desktop environments have applications that wake up and do rendering
every 1-3 seconds, even when the screen is off and the machine is
completely idle.

But when I was testing my PC8+ patches I set the default value to
100ms so I could use the bad-behaving desktop environments to
stress-test my patches. I also thought it would be a good idea to ask
our power management team to test different values, but I'm pretty
sure they would ask me for an easy way to change the timeout. So to
help these 2 cases I decided to create an option that would make it
easier to change the default value. I also expect people making
specific products that use our driver could try to find the perfect
timeout for them.

Anyway, fixing the bad-behaving applications will always lead to
better power savings than just changing the timeout value: you need to
stop waking the Kernel, not quickly put it back to sleep again after
you wake it for nothing. Bad sleep leads to bad mood!

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>