[PowerPC] Revert P9 scheduling model to incomplete

Partially revert a previous change from commit: https://llvm.org/svn/llvm-project/llvm/trunk@314026
The previous change caused regressions on Power 9.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314835 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed