msm: kgsl: Move ringbuffer start to target specific code

Currently the ringbuffer start sequence is a mix between generic and
target specific code (and some target specific behavior mixed into the
generic for good measure).  Move all the behavior to the target specific
code which duplicates a bit of code but increases our flexibility for
the future.

Change-Id: Ic0dedbadf0aa9c2af8acfde481f2b62d991f8c1e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>