msm: kgsl: enable CPU IRQ while suspending context

Make sure that CPU interrupt is enabled before calling adreno
suspend_context when moving from NAP/SLEEP to SLUMBER state.
We send commands to GPU in adreno_drawctxt_switch() which will generate
GPU interrupt. Since CPU interrupt is not enabled, adreno_idle() will
fail because pending interrupt will not be served by CPU this causes
false GPU hang.

CRs-Fixed: 547813

Change-Id: Ia4b730b8f8cda1e820f571ffb7ddb00336ca1a8c
Signed-off-by: Gaurav Gagrani <ggagrani@codeaurora.org>