i915: Fix spurious TV detection after 9d0498a2 + 9559fcdb

Partial revert of 9d0498a2.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Tested-by: Hugh Dickins <hughd@google.com>
Tested-by: Sven Joachim <svenjoac@gmx.de>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>