drm/i915: set TV detection mode when tv is already connected

We used load_detect_temp flag to determine whether to set tv to the test
mode. However if the TV already has a mode set, we still need to set the
test mode to determine connection.  This results in blinking, but there is
no other reliable way to determine TV connection.

freedesktop.org bug #22035

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>