msm: kgsl: Poll GDCSR for holi target to ensure CX collapse

The regulator_is_enabled() API doesn't guarantee that CX gdsc has
collapsed at hardware. There could be a vote on the GDSC from another
subsystem like TZ. So poll the CX GDSCR register to ensure that CX has
indeed collapsed.

Change-Id: Ic07cde5ba3442689cb73b33fe57c3d4371cb9565
Signed-off-by: Puranam V G Tejaswi <pvgtejas@codeaurora.org>