drm/bridge: dw-hdmi: Return number of EDID modes

The dw_hdmi_connector_get_modes() function accidentally forgets to
return the number of modes it added, although it has this information
stored in a local variable.  Let's fix that.

Without this fix, drm_helper_probe_single_connector_modes_merge_bits()
could get confused and always call drm_add_modes_noedid().  That's not
right.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>