R600: Move Subtarget feature definitions into AMDGPU.td

This is the convention used by the other targets.

Reviewed-by: Vincent Lejeune <vljn@ovi.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183559 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed