intel/elk: Remove code for Gfx9+
On top of !27563 (merged) and !27770 (merged).
This removes dead code from ELK since Gfx9+ is not used there. There might be still more code that can be simplified, but there's a point of diminished returns to clean up ELK.
The series of smaller commits should help reviewing, and bisecting future regressions. Let me know if it is preferred to squash it into a single change.