drm/probe-helper: add drm_connector_helper_get_modes()

Add a helper function to be used as the "default" .get_modes()
hook. This also works as an example of what the driver .get_modes()
hooks are supposed to do regarding the new drm_edid_read*() and
drm_edid_connector_update() calls.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d985449ed4b95971490ab7c09d2d59b58a892769.1656494768.git.jani.nikula@intel.com