drm/i915/tc: Check for DP-alt, legacy sinks before taking PHY ownership

On ADL-P the PHY ready/complete flag is always set even in TBT-alt mode.
To avoid taking the PHY ownership and the following spurious "PHY sudden
disconnect" messages on this platform when connecting the PHY in TBT
mode, check if there is any DP-alt or legacy sink connected before
taking the ownership.

v2: (Jose)
- Fix debug message clarifying that a TBT sink can be connected.
- Add comments describing the PHY complete HW flag semantic differences
  between adl-p and other platforms.

Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929132833.2253961-2-imre.deak@intel.com