drm/amdgpu/vcn: set the priority for each encode ring

VCN has multiple rings. Set the proper priority level for each
encode ring while initializing.

Signed-off-by: Satyajit Sahu <satyajit.sahu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>