msm: kgsl: Cleanup GPU regulators

We only ever have at most two regulators, and sometimes only one.
We don't need a complex set of lists and loops when only two pointers
will do.  Furthermore, explicitly forcing the regulators into the pwrctrl
struct helps ensure that they get enabled and disabled in the correct
order.

To assist, we add a new helper function called kgsl_regulator_disable_wait
that consolidates code in the driver used to wait for regulators to finish
disabling.

Change-Id: Ic0dedbad3aaa9c5d2f75dd33ee4e368eceb91d34
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>