drm/i915/bios: Use the platform's port_mask when there is no VBT

When we have no VBT we currently assume ports A-F are
all pontially valid for every platform. That is nonsense.
Grab the bitmask of valid ports from the runtime info
instead.

Although the defaults we actually fill here look semi-sensible
only for hsw-skl era hardware. Dunno if we should try to do
something more appropriate here for other platforms,
or just try to nuke the whole thing?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240319092443.15769-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>