msm: kgsl: Only read throttle counters if GMU is enabled

Throttle counters are only used with GMU.
Platforms without the GMU should return early
without reading them.

Change-Id: I0df614439c0629afbc7a1ebaca279e4a1042949d
Signed-off-by: Sushmita Susheelendra <ssusheel@codeaurora.org>