drm/i915: Add encoder hook to get the PLL type used by TC ports

Add an encoder hook, which can be called on enabled TC ports to
determine if the port uses a TBT or a non-TBT PLL. An upcoming patch
will use this to sanity check active TC port's PHY state wrt. the PLL
type used by the port.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316131724.359612-13-imre.deak@intel.com