drm/amdgpu: Use DRM scheduler API in amdgpu_xcp_release_sched

Lets use the existing helper instead of peeking into the structure
directly.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>