drm/ast: Move DP501-based connector code into separate helpers

Add helpers for DP501-based connectors. DP501 provides output via
DisplayPort. This used to be handled by the VGA connector code.

If a DP501 chip has been detected, ast will now create a DisplayPort
connector instead of a VGA connector.

Remove the DP501 code from ast_vga_connector_helper_get_modes(). Also
remove the call to drm_connector_update_edid_property(), which is
performed by drm_get_edid().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-9-tzimmermann@suse.de