msm: kgsl: Avoid out of bound register access for A6xx

Avoid GMU register access when gmu core is disabled.
This change handles warning messages occurred during
register access when GMU is in disabled state.

Change-Id: I6ea9bfaf3ce6d239e0b2b3e8babfaa29bc7053a2
Signed-off-by: Amit Kushwaha <kushwaha@codeaurora.org>