msm: kgsl: Remove gmu_core_regulator_isenabled()

Commit 0d44684c ("msm: kgsl: Take GMU snapshot on GMU failures")
removed the only usage of gmu_core_regulator_isenabled().

Change-Id: Ic0dedbadb40e1faed4a9e0629e9d0babb718f6b3
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>