kgsl_pwrctrl.c: change pwr->default_pwrlevel

- ...to pwr->min_pwrlevel to allow SLEEP/SLUMBER/SUSPEND states on lower
frequency
- see:

https://android.googlesource.com/kernel/msm/+/cb8b054a8bc417f48ff62468bd257df339d6e9be%5E1..cb8b054a8bc417f48ff62468bd257df339d6e9be/

Signed-off-by: SkipXS <sskkiipp@gmail.com>