drm/i915: Add bit field to record which pins have received HPD events (v3)

This way it is possible to limit 're'-detect() of displays to connectors
which have received an HPD event.

v2: Reordered drm_i915_private: Move hpd_event_bits to hpd state tracking.
v3: Fixed merge conflicts with previous patches.

Signed-off-by: Egbert Eich <eich@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>