msm: kgsl: Do not check for hang when graphics core is idle

GPU hang detection logic checks register values every 2 seconds
and determines a hang if there is no change in the register values
after 2 seconds. When GPU core is idle the register values do not
change, hang detection logic should be disabled to prevent
false hangs being detected.

CRs-fixed: 402818
Change-Id: I6bd9085879fcafda9ca322221ba93e6a6ca5043c
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>