PM: devfreq: Fixup device transition and usage statistics

Ignore bogus data during slumber in transition and usage stats that
rendered the output basically useless. Also fix the reported GPU
frequency.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
PM: devfreq: Use high priority workqueue

It does not make sense to run kgsl on high and devfreq on regular
priority.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
PM: devfreq: Adapt devfreq_get_freq_level to cpubw requirements

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>