drm/amdgpu: add plumbing for ctx priority changes v2

Introduce amdgpu_ctx_priority_override(). A mechanism to override a
context's priority.

An override can be terminated by setting the override to
AMD_SCHED_PRIORITY_UNSET.

v2: change refcounted interface for a direct set

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>