msm: kgsl: Move voltage levels to the GPU specific powerlevels

There are only two reasons for us to define a OPP table these days, and
one of them is to define the voltage levels that we send to the GMU.
That is silly because we have a perfectly good power level definition
already handy. Move the levels to the GPU definition and use them
accordingly.

Change-Id: Ic0dedbad2d3c84a2de7cda5311ff99e96434b9e5
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>