msm: kgsl: Do not change active_pwrlevel while entering SLUMBER

Active power level shouldn't be set to level corresponding to XO
clock while entering SLUMBER. Any change in active_power level
should happen from kgsl_pwrctrl_pwrlevel_change and it should
always be less than or equal to min_pwrlevel and greater than
or equal to max_pwrlevel.

Change-Id: I8b7013a6cea7544e220026b59ea5405377509f18
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>