msm: kgsl: Make GPU core support more generic

GPU device code has mostly congealed into place over time. Remove
some of the nastier workarounds and try to provide a consistant
method for accessing the GPU core specific functions and bits
and pieces located therein. Add more feature flags to avoid
if (adreno_is_XXX()) decision trees and try to not convert back
and forth between struct kgsl_device and struct adreno_device so
much.

Change-Id: Ic0dedbad3692f6d1f4db535ec74178203d3aab53
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>