drm/mgag200: Use drm_connector_helper_get_modes()

Mgag200's .get_modes() function is identical to the common helper.
Use the latter.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240513125620.6337-11-tzimmermann@suse.de