[AMDGPU] Fix getEUsPerCU for gfx10 in CU mode

Summary:
"Per CU" is a bit simplistic for gfx10, but I couldn't think of a better
name.

Reviewers: arsenm, rampitec, nhaehnle, dstuttard, tpr

Subscribers: kzhuravl, jvesely, wdng, yaxunl, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76861