drm/amdgpu: add interface for editing a foreign process's priority v3

The AMDGPU_SCHED_OP_PROCESS_PRIORITY_OVERRIDE ioctls are used to set
the priority of a different process in the current system.

When a request is dropped, the process's contexts will be
restored to the priority specified at context creation time.

A request can be dropped by setting the override priority to
AMDGPU_CTX_PRIORITY_UNSET.

An fd is used to identify the remote process. This is simpler than
passing a pid number, which is vulnerable to re-use, etc.

This functionality is limited to DRM_MASTER since abuse of this
interface can have a negative impact on the system's performance.

v2: removed unused output structure
v3: change refcounted interface for a regular set operation

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>