drm/msm/a6xx: Remove unwanted regulator code

The GMU code currently has some misguided code to try to work around
a hardware quirk that requires the power domains on the GPU be
collapsed in a certain order. Upcoming patches will do this the
right way so get rid of the unused and unwanted regulator
code.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>