[AMDGPU] Fix CS scratch setup on pre-GCN3 ASICs

Summary:
Prior to GCN3 s_load_dword offsets are in dwords rather than bytes.
Thus the scratch buffer descriptor offset must be adjusted for pre-GCN3 ASICs.

Reviewers: nhaehnle, tpr

Reviewed By: nhaehnle

Subscribers: sheredom, arsenm, kzhuravl, jvesely, wdng, yaxunl, dstuttard, t-tye, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D56496

llvm-svn: 353530