drm/i915/tgl: move DP_TP_* to transcoder

Gen 12 onwards moves the DP_TP_* registers to be transcoder-based rather
than port-based. This adds the new register addresses and changes all
the callers to use the register saved in intel_dp->regs.*. This is
filled out when preparing to enable the port so we take into account if
we should use the transcoder or the port.

v2: reimplement by stashing the registers we want to access under
intel_dp->reg. Now they are initialized when enabling the port.
Ville suggested to store the transcoder to be used exclusively
by TGL+. After implementing I thought just storing the register directly
made it cleaner.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190904213419.27547-5-jose.souza@intel.com
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>