msm: kgsl: Poll GDSCR 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: Id98c5318d5358b16f4277cb5d96027add63ad801
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>