msm: kgsl: Let normal register functions access GMU

Update the kgsl register read and write functions to also access the
GMU register range with suitable range checking. This will simplify
common code that accesses both GMU and non-GMU registers.

Change-Id: Iea02c721b4cf6cdbc42ecdd72a490cca34a261ee
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>