drm/i915: Enable hotplug polling after registering the outputs

Previously we would enable hotplug polling on the outputs immediately
upon construction. This would allow a very early hotplug event to
trigger before we had finishing setting up the driver to handle it.
Instead, move the output polling to the last step of registration, after
we have set up all handlers, including the fbdev configuration.

v2: Symmetrically turnoff the hotplug helper in unregister after the
fbdev is first synchronised then finalized. This stops a late hotplug
event being processed after the interrupts are disabled.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20171128110147.28654-1-chris@chris-wilson.co.uk