drm/amd/display: Add a clock source to a sharing pipe

Fix to allow DVI displays to share the same clock source

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>