drm/i915: Eliminate PORT_HOTPLUG_EN setup from gen3/4 irq_postinstall

We've already cleared PORT_HOTPLUG_EN in the .irq_preinstall hook
so doing it again in the .irq_postinstall is pointless.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-8-ville.syrjala@linux.intel.com