drm/i915: Add port A HPD support for IVB/HSW

As with ILK/SNB wire up the port A HPD on IVB/HSW.

This might be more important on HSW with PSR. BSpec tells us that if the
automagic link training performed by the hardware fails for some reason,
we're going to get a short HPD and are supposed to re-train the link
manyally.

v2: 0 initialize pin_mask/long_mask due to intel_get_hpd_pins() changes
    Add a comment about the pulse duration bits being reserved on HSW+
    like we have for LPT+ in ibx_hpd_irq_setup()

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>