msm: kgsl: constify structs with function pointers

Now that the last of the runtime modifications to adreno_gpudev are gone
finally mark it and the other structs as const to protect the function
pointers within.

Change-Id: Ic0dedbad25b39b5df0fdd565a494164e1588c9a1
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>