msm: kgsl: Move gpu_idle_level_names

gmu_idle_level_names is only used in one place in one file. It can be
removed from the header file and replaced with a simple static function.

Change-Id: Ic0dedbad45c4f177979d4f8bcef7a7f2b60dd0a7
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>