msm: kgsl: Skip state change in idle check, if requested state is NONE

After scheduling the idle work for transitioning to either NAP or SLUMBER
it's possible that requested state can change to NONE if any new workload
comes before kgsl_idle_check is executed or it gets the device mutex.
If this happens, current code calls pwrctrl_change_state which fails as
this is not a valid transition. To prevent this, skip calling
pwrctrl_change_state if requested state is NONE.

Change-Id: Iae535c6e2a3230f9e7e210712990eeb405822f4f
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>