msm: kgsl: Return correctly from gmu_core_dev_wait_for_lowest_idle

Return value from a6xx_gmu_wait_for_lowest_idle is not
handled properly in its core function. Return correctly
from core function to handle error properly.
This will help to trigger GMU_FAULT and collect gmu
snapshot from gmu_stop or adreno_stop in case of any
idle timeout.

Change-Id: Iedffac031d11bf8eebadfcb10857c4e99142037f
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>