drm/i915/tgl: Add additional PHYs for Tiger Lake

Tiger Lake has up to 3 combo phys and 6 TC phys. Extend the helper
conversion functions from port to phy.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190711173115.28296-14-lucas.demarchi@intel.com