msm: kgsl: Use correct checks for GPMU related functionality

Use "gmu_core_gpmu_isenabled" check for all GPMU related
functionality instead of "gmu_core_isenabled" as later will
return true even when ADRENO_GPMU feature flag is not defined.

Change-Id: Ic577052d8c1cc52965e07c557b178fec444c3613
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>