drm/mgag200: convert get modes to struct drm_edid

Convert mgag200_vga_connector_helper_get_modes() to use struct drm_edid
based functions directly.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/19a453c725fc27bd890f8fc73104f43a376dfce0.1704473654.git.jani.nikula@intel.com