drm/i915/ddi: Silence compiler warning for unknown output type

Silences

	src/drivers/gpu/drm/i915/intel_ddi.c: warning: 'port' may be used uninitialized in this function [-Wuninitialized]

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1459717154-27607-1-git-send-email-chris@chris-wilson.co.uk