msm: kgsl: Create a6xx gmu power ops

Instead of overwhelming the legacy power up/down sequences,
create a6xx gmu specific power up/down sequences. The
gpudev power ops will be set during a6xx gmu probe so that
we don't have to do gmu checks in legacy probe.

Change-Id: I897d775bdc3c9f97d38d20bc39b56f2551703bce
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>