drm/mgag200: Set .detect_ctx() and enable connector polling

Set .detect_ctx() in struct drm_connector_helper_funcs to the
common helper drm_connector_helper_detect_from_ddc() and enable
polling for the connector. Mgag200 will now test for the monitor's
presence by probing the DDC in regular intervals.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610141141.29527-4-tzimmermann@suse.de