msm: kgsl: Don't BUG_ON when entering DEEP NAP

Deep Nap is entered when the Deep Nap timer expires and
a work is scheduled. There can be enough latency before the
worker thread executes and the device can enter ACTIVE state
in the meanwhile. Hence gracefully exit instead of BUG_ON.

Change-Id: Iaec6be3c34d038774750800de2bac79052176202
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>