drm/i915/bios: Document the mess around the LFP data tables

Document the fact that struct lvds_lfp_data_entry can't be used
directly and instead must be accessed via the data table pointers.

Also remove the bogus comment implying that there might be a
variable number of panel entries in the table. There are always
exactly 16.

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/20220504150440.13748-5-ville.syrjala@linux.intel.com