msm: kgsl: Fix KGSL_PROP_PWRCTRL property for gmu based targets

This property is used to enable dcvs or disable both dcvs and slumber
using kgsl power state transitions. Since gmu or rgmu based targets
don't use the power states, add and use gmu core flag to enable and disable
slumber for those targets.

Change-Id: Iace3af9427c7b84509c9041604972cf14e7cd675
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>