drm: add drm_connector_attach_edid_property()

drm_connector_init doesn't attach the edid property for some connector
types, drm_connector_attach_edid_property() can be used to enable the
edid property in these cases.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20181002111041.17053-2-kraxel@redhat.com