drm/panel: panel-simple: add default connector_type

All panels shall report a connector type.
panel-simple has a lot of panels with no connector_type,
and for these fall back to DPI as the default.

v2:
  - Rebased on top of validation of panel description

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200726203324.3722593-3-sam@ravnborg.org