drm/amd/display: Make DCN stream encoder shareable

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>