drm/probe-helper: Add helper for drm_helper_probe_single_connector_modes()

The drm_helper_probe_single_connector_modes() is a bit long. Let's
break a chunk off to update and validate modes. This helps avoid one
goto and also will allow us to more easily call the helper a second
time in a future patch without adding looping or another goto.

This change is intended to be a no-op change--just code movement.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220511155749.v3.1.I2dd93486c6952bd52f2020904de0133970d11b29@changeid