msm: kgsl: Consolidate some gmu functions

Move some gmu functions to gmu_core so that
those can reusable without reimplementing on
other kind of gmu.

Change-Id: Ie083bd541f716eabf56da8cef4e7606a2595d482
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>