drm/i915: Wire up port A aux channel

Useful for dp aux to work better. Also stop enabling the port A
hotplug event - eDP panels are expected to fire that interupt and
we're not really ready to deal with them. This is consistent with how
we handle port A on ilk-hsw.

The more important bit is that we must delay the enabling of hotplug
interrupts until all the encoders are fully set up. But we need irq
support earlier than that, hence hotplug interrupts can only be
enabled in the ->hpd_irq_setup callback.

v2: Drop the _HOTPLUG, it isn't (Ville).

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