drm/i915: Add port A HPD support for ILK/SNB

ILK/SNB support port A HPD. While HPD is optional on eDP let's at least
try to wite it up so that we might notice if the link has issues.

The eDP spec suggests that if HPD is not wired up, one should poll the
link status instead. We don't even do that currently.

v2: 0 initialize pin_mask/long_mask due to intel_get_hpd_pins() changes

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>