msm: kgsl: Move rscc to a6xx gmu space

Rscc registers are only needed for gmu based targets. So move the
probe and snapshot to a6xx gmu probe and snapshot respectively.
Also, fix the rscc offset for targets which have rscc registers
part of the gmu register space.

Change-Id: I3f73e3c99838d01676f81df6bc7bd7ab3334a0fe
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>