msm: kgsl: Avoid slumber if interrupt is pending

As part of slumber sequence, there is a hardware idle validation
which checks for GPU idle. However, it is possible that GPU is idle
but the interrupt is yet to be processed. Fix it by checking for
pending interrupt.

Change-Id: Ia3aef2827fc48bfad382e2369d8477aa67b163d9
Signed-off-by: Kamal Agrawal <kamaagra@codeaurora.org>