drm/i915: fix short vs. long hpd detection

Fix short vs. long hpd detection for non-g4x and non-pch split
platforms.

Broken since introduction in
commit 13cf5504
Author: Dave Airlie <airlied@redhat.com>
Date:   Wed Jun 18 11:29:35 2014 +1000

    drm/i915: rework digital port IRQ handling (v2)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83175


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>