msm: kgsl: Fix adreno idle detection during hang

Adreno idle detection during a hang was being obstructed when
called from adreno_waittimestamp(). The variable active_cnt which
was meant to protect the device from going into suspend while
doing a waittimestamp was being set and was preventing the
check of rbbm_status register which was leading to a false hang
detection, where, if the rbbm_status register value had been
probed the device would have been found to be idle. We fix this by
having a separate function is_adreno_rbbm_status_idle() which checks
this register and report whether the core is idle or not. And we call
this function instead of the generic adreno_isidle() inside
adreno_hang_detect(). Doing this fixes at least one scenario of false
hang detection.

CRs-fixed: 406631
Change-Id: Ibe80a91513af184bd95c9a710fa696432544621a
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>