msm: kgsl: Temporarily mask the watchdog upon receiving it

Once the GMU watchdog interrupt is cleared and the handler
returns, if the device has not been reset the interrupt
will fire again. This can cause storms of GMU watchdog
interrupts that flood the system when something goes wrong.

Mask the GMU watchdog interrupt when we receive it, so that
we do not receive it again before we finish recovering. It
will be unmasked again the next time the GMU boots.

Change-Id: Ieb9090f9ed274d581f706dced683f811c66bba70
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>