drm/i915: Fix hotplug interrupt enabling for SDVOC

A broken conditional would lead to SDVOC waiting upon hotplug events on
SDVOB - and so miss all activity on its SDVO port.

This regression has been introduced in

commit 1d843f9d
Author: Egbert Eich <eich@suse.de>
Date:   Mon Feb 25 12:06:49 2013 -0500

    DRM/I915: Add enum hpd_pin to intel_encoder.

References: https://bugs.freedesktop.org/show_bug.cgi?id=58405


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add regression note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>