drm/i915: extract connector update from intel_ddc_get_modes() for reuse

Refactor the connector update part of intel_ddc_get_modes() into a separate
intel_connector_update_modes() function for reuse. No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=45881


Tested-by: Alex Ferrando <alferpal@gmail.com>
Cc: stable@vger.kernel.org (for 3.4+3.5)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>