msm: kgsl: Use IDLE interrupt to switch to NAP state

Replace busy waiting loop to interrupt driven.
Current code starts probing for GPU idle after
RB_INT interrupt. This prevents CPU
from clock gating for 150-400uS per frame
and burns battery.

This change utilizes the RBBM_GPU_IDLE interrupt from
GPU. The GPU triggers this interrupt when it is
entering IDLE, so there's no need for busy
probing loop.

Change-Id: Ib00780ff426927a6e88013804d9b08a63e06affd
Signed-off-by: Oleg Perelet <operelet@codeaurora.org>