msm: kgsl: Allow state transition to nap from valid states

Currently transitions to nap state from active, slumber or aware
are not permitted. This fix allows transition from active or slumber
to nap state.

Change-Id: I889c050fb7042fefcab40ea4fa84efabef2149e8
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>