drm/amd/display: fix the clock source contruct for dcn315

[why]
Driver sends invalid deep color ratio to DMUB. Update it to the right
one by using another clock source construct.

Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>