drm/i915: only enable hotplug for detected outputs

This patch changes around our hotplug enable code a bit to only enable
it for ports we actually detect and initialize.  This prevents problems
with stuck or spurious interrupts on outputs that aren't actually wired
up, and is generally more correct.

Fixes FDO bug #23183.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>