drm/i915/dg1: provide port/phy mapping for vbt

As with RKL, DG1's VBT outputs are indexed according to PHY rather than
DDI.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201007002210.3678024-8-lucas.demarchi@intel.com