msm: kgsl: Add 8974 support for enabling GPU GDHS

We add 8974 support for enabling GDHS of GPU power rail line
by adding it to the 8974 device tree. This feature allows
GPU to go in SLUMBER instead of SLEEP state as 8974 has a low
latency power rail bring-up.

Change-Id: I4c098947c50277817af997e0f8cc7ec232bc42c9
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>