[AMDGPU] Add file headers for few files where it is missing.

Summary:
Added file headers for files which implement iterative lightweight scheduling
strategies. Which is basically an exercise which I undertook in order to get
used to LLVM development process.

Reviewers: arsenm, vpykhtin, cdevadas

Reviewed By: vpykhtin

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, javed.absar, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73417