drm/i915/tc: Check TC mode instead of the VBT legacy flag

After the previous patch the TC mode in the connect/disconnect functions
is always in sync with the VBT legacy port flag, so for consistency with
the rest of the function check the TC mode instead of the VBT flag.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-16-imre.deak@intel.com