drm/i915/bios: Reorder panel DTD parsing

Reorder things so that we can parse the entier LFP data block
in one go. For now we just stick to parsing the DTD from it.

Also fix the misleading comment about block 42 being deprecated.
Only the DTD part is deprecated, the rest is still very much needed.

v2: Move the version check+comment into parse_generic_dtd() (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504150440.13748-2-ville.syrjala@linux.intel.com