drm/amdgpu: share scheduler score on VCN3 instances

The VCN3 instances can do both decode as well as encode.

Share the scheduler load balancing score and remove fixing encode to
only the second instance.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>