msm: kgsl: Fix gpu idle check

Fix the mask being used to check whether gpu has a pending
irq or not and check whether gpu status is idle before checking
for a pending irq. If pending irq is checked before the gpu
status then there is a possibility of the irq being set after
it has been checked but before the gpu status is checked which
will result in the logic detecting that gpu is idle with a
pending irq.

Change-Id: I5f97b6f3f4c6b58c95e91604ae87330d06685962
CRs-Fixed: 647176
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>