Refactor edp dsc codes.

We split out edp dsc config from "global" to "per-panel" config settings.
Reviewed-by: Mike Hsieh <mike.hsieh@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Ian Chen <ian.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>