drm/i915/skl: enable DDI-E hotplug

v2: fix one error found by checkpath.pl
v3: Add one ignored break for switch-case. DDI-E hotplug
    function doesn't work after updating drm-intel tree,
    I checked the code and found this missing which isn't
    the root cause for broke DDI-E hp.  The broken
    DDI-E hp function is fixed by "Adding DDI_E power
    well domain".

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>