drm/i915: Introduce HPD_PORT_TC<n>

Make a clean split between hpd pins for DDI vs. TC. This matches
how the actual hardware is split.

And with this we move the DDI/PHY->HPD pin mapping into the encoder
init instead of having to remap yet again in the interrupt code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630215601.28557-11-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>