KGSL: make simple govenor a selectable option, co-exist with ondemand

also removed some hardcoded values

Signed-off-by: Paul Reioux <reioux@gmail.com>

	modified:   drivers/gpu/msm/kgsl_pwrscale.c
	modified:   drivers/gpu/msm/kgsl_pwrscale_trustzone.c