msm: kgsl: Separate out reset sequence for a6xx gmu targets

Now that we have a6xx gmu specific gpudev, we can dissociate the
gmu reset sequence from the generic a6xx_reset function.

Change-Id: Ib56456d5390fc059fc34407941f805c06cdfe42a
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>