drm/i915/dp: Do not switch aux to TBT mode for non-TC ports

Non-TC ports always have tc_mode == TC_PORT_TBT_ALT so it was
switching aux to TBT mode for all combo-phy ports, happily this did
not caused any issue but is better follow BSpec.
Also this is reserved bit before ICL.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Fixes: e9b7e142

 ("drm/i915: Sanitize the terminology used for TypeC port modes")
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191029011014.286885-1-jose.souza@intel.com