msm: kgsl: Remove nap_allowed variable and functionality

Allow NAP on all targets.  The splitting of the clock enable
and disable calls into enable/prepare and disable/unprepare
allows us to safely make this change for all targets.

Change-Id: I03d909b86aef33631a887d159cf0a807a6d0ae75
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>