msm: kgsl: Move APRIV config to a6xx start

We need APRIV enabled for eCP as well. But eCP doesn't
call into a6xx_rb_start. Move it to a more generic function
which is called by eCP as well.

Change-Id: I3014bee48145ab5b7c54f01854fe520d9a6db0a6
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>