msm: kgsl: Use regulator_is_enabled api when gpu-quirk-cx-gdsc is defined

When gpu-quirk-cx-gdsc is defined, cx_gdsc will not be disabled from
HLOS, so use regulator_is_enabled api to get the dummy status of cx_gdsc.

Change-Id: Ied2a54687d9438610116bf4a96a4843fdbc05c56
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>