drm/i915: Really ignore long HPD pulses on eDP

Return IRQ_HANDLED from intel_dp_hpd_pulse() to properly
ignore the long HPD pulse on eDP to avoid the never ending
VDD off->HPD->VDD on->VDD off->HPD... cycle.

This fixes a regression intoduced by
 commit b2c5c181
 Author: Daniel Vetter <daniel.vetter@ffwll.ch>
 Date:   Fri Jan 23 06:00:31 2015 +0100

    drm/i915: Use symbolic irqreturn for ->hpd_pulse

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>