msm: kgsl: Remove IRQ on/off sequence from nap state

Do not disable/enable irq when device goes into and comes out of
nap. Instead, move the disable/enable to when the device goes
into and comes out of slumber. This reduces the nap latency
and increases performance.

CRs-Fixed: 730433
Change-Id: I00f209d4049c4e8d354e3656198f5acc2221c958
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>