PM: devfreq: Add conservative governor

A GPU governor is not rocket science. This one gets the job done pretty
smoothly while complying with devfreq framework rules.

Change-Id: Ieb986882b9972ef36c488ca814a15a944c131c6b
PM: devfreq: Add some conservativeness

Change-Id: I456c0da8103c8cfa8c3c5241d3ac0c3d7facadf8
PM: devfreq: Always reflect a change of polling interval

Change-Id: I432b0f858b0db90a9fd47afaa9d602593179692d
PM: devfreq: conservative: Return 0 if GPU just started

Bailing out early is perfectly fine, so do not trigger an error in
update_devfreq.

Change-Id: I5357298db7c4452f267d636afb2fdce30c754f49