drm/i915: Don't touch PORT_HOTPLUG_EN in intel_dp_detect()

PORT_HOTPLUG_EN has allready been setup in i915_driver_irq_postinstall(),
when intel_dp_detect() runs.

Delete the DP[BCD]_HOTPLUG_INT_EN defines, they are not referenced anymore.

I found this while searching for a fix for
        https://bugzilla.redhat.com/show_bug.cgi?id=528312



Signed-off-by: Karsten Wiese <fzu@wemgehoertderstaat.de>
Signed-off-by: Eric Anholt <eric@anholt.net>