drm/i915/mg: Use tc_port instead of port parameter to MG registers

All the MG registers is based on the tc_port not port, so
MG_PHY_PORT_LN() was subtracting port and PORT_C what is very
fragile.
So replacing port to tc_port in all MG register macros and users
like we have for DKL.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191001193729.123736-1-jose.souza@intel.com