amdgpu: Add context priority override function.

This way we can override the priority of a single context using a
master fd.

Since we cannot usefully create an amdgpu device of a master fd
without the fd deduplication kicking in this takes a plain fd.

This can be used by e.g. radv to get high priority contexts using
a master fd from the primary node or a lease.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>