drm/panel-edp: Avoid adding multiple preferred modes

drm/panel-edp: Avoid adding multiple preferred modes

If a non generic edp-panel is under aux-bus, the mode read from edid would
still be selected as preferred and results in multiple preferred modes,
which is ambiguous.

If both hard-coded mode and edid exists, only add mode from hard-coded.

Signed-off-by: Hsin-Yi Wang <[email protected]>
Reviewed-by: Douglas Anderson <[email protected]>
Signed-off-by: Douglas Anderson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]