drm/i915: add VGA hotplug support for 945+

Add VGA port hotplug detection to the i915 driver.  When KMS is enabled,
plugging in or removing a VGA cable from the VGA connector will
generate a uevent, which indicates to userspace that it should re-probe
outputs on this device (to determine modes, etc.).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[anholt: dropped extra PORT_HOTPLUG_STAT clear with ack from jbarnes]
Signed-off-by: Eric Anholt <eric@anholt.net>