drm/i915/skl: handle port E in cpt_digital_port_connected

drm/i915/skl: handle port E in cpt_digital_port_connected

SKL port E handling was added in

commit a829e7c40b431e3e453ac7b2ad3cb8e734a67b81
Author: Xiong Zhang <xiong.y.zhang@intel.com>
Date:   Mon Aug 17 15:55:50 2015 +0800

    drm/i915/skl: enable DDI-E hotplug

but the whole function was moved in a another branch in

commit 5ded74f9e07cbe1039026b757c7b5c10d40fe560
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Aug 20 10:47:36 2015 +0300

    drm/i915: move ibx_digital_port_connected to intel_dp.c

and the addition was lost at some backmerge that I was unable to
identify. Put it back in.

Tested-by: Tomi Sarvela <tomix.p.sarvela@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>