[AMDGPU] Add ML-oriented coexec scheduler selection and queue handling (#169616)
This patch adds the initial coexec scheduler scaffold for machine
learning workloads on gfx1250.
It introduces function and module-level controls for selecting the
AMDGPU preRA and postRA schedulers, including an `amdgpu-workload-type`
module flag that maps ML workloads to coexec preRA scheduling and a nop
postRA scheduler by default.
It also updates the coexec scheduler to use a simplified top-down
candidate selection path that considers both available and pending
queues through a single flow, setting up follow-on heuristic work.