[AMDGPU] Prevent backend override of WGP when using PAL
[AMDGPU] Prevent backend override of WGP when using PAL

Summary:
Backend defaults to setting WGP mode to 0 or 1 depending on the cumode
feature. For PAL clients we want PAL to set this mode.
Adding check to prevent the backend overriding whatever the front end has set.

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63639

Change-Id: Ia3bac3e562fb47089f05c11cae47d1c92f7195ee

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm