PM: devfreq: Fix simple_ondemand crashing on startup

simple_ondemands private data must be set to NULL, otherwise we would
run into a NULL pointer in kgsl_devfreq_get_dev_status().

Change-Id: I6cc6a8b11e3b58b8c3e3c26d43ee36949cf62351
ARM/dt: msm8974-gpu: Enable qcom bus-control

Honor DEVFREQ_FLAG_FAST/SLOW_HINT flags in adreno-tz.

Change-Id: I5b995fb29b690946651304ed1607214c63485abc