[llvm] [AMDGPU][SIInsertWaitcnts][NFC] Drop `AMDGPU::` (PR #180663)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > A previous PR #178345 introduced `using namespace llvm::AMDGPU`, so this patch drops `AMDGPU::`.
> 
> I'd probably go for dropping the using namespace

That was [my initial suggestion](https://github.com/llvm/llvm-project/pull/178345#discussion_r2758084562) too.

https://github.com/llvm/llvm-project/pull/180663