drm/i915/tgl: Add DPLL registers

On TGL the port programming for combophy is very similar to ICL, so
adapt the callers to possibly use the different register values.

v2 (Lucas): Add TODO with about DPLL4 (requested by Ville)

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190711173115.28296-21-lucas.demarchi@intel.com