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.

Conflicts:
	drivers/devfreq/Kconfig
	drivers/devfreq/Makefile

Change-Id: Iffdd5106bc419e85de2fee9d7606a0255332a445
PM: devfreq: Add some conservativeness
PM: devfreq: Always reflect a change of polling interval
PM: devfreq: conservative: Return 0 if GPU just started

Bailing out early is perfectly fine, so do not trigger an error in
update_devfreq.
msm: hotplug: Remove deprecated msm_cpufreq_set_freq_limits
defconfig: Enable conservative governor

Change-Id: I442051fd4c2f23c1929c5802108c69df24c5c8d6