drm/mgag200: Implement connector's get_modes with helper

Provide drm_connector_helper_get_modes_from_ddc() to implement the
connector's get_modes callback. The new helper updates the connector
from DDC-provided EDID data.

v2:
	* clear property if EDID is NULL in helper

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220516134343.6085-4-tzimmermann@suse.de