drm/amd/display: Use parameter for call to set output mux

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>