drm/i915/vbt: Handle generic DTD block

VBT revision 229 adds a new "Generic DTD" block 58 and deprecates the
old LFP panel mode data in block 42.  Let's start parsing this block to
fill in the panel fixed mode on devices with a >=229 VBT.

v2:
 * Update according to the recent updates:
    - DTD size is now 16 bits instead of 24
    - polarity is now just a single bit for hsync and vsync and is
      properly documented
 * Minor checkpatch fix

v3:
 * Now that panel options are parsed separately from the previous patch,
   move generic DTD parsing into a function parallel to
   parse_lfp_panel_dtd.  We'll still fall back to looking at the legacy
   LVDS timing block if the generic DTD fails.  (Jani)
 * Don't forget to actually set lfp_lvds_vbt_mode!  (Jani)
 * Drop "bdb_" prefix from dtd entry structure.  (Jani)
 * Follow C99 standard for structure's flexible array member.  (Jani)

v4:
 * Add "positive" to polarity field names for clarity.  (Jani)
 * Move VBT version check and fallback to legacy DTD parsing logic to a
   helper to keep top-level VBT parsing uncluttered.  (Jani)
 * Restructure reserved bit packing at end of generic_dtd_entry from
   "u32 rsvd:24" to "u8 rsvd[3]" to prevent copy/paste mistakes in the
   future.  (Jani)

Bspec: 54751
Bspec: 20148
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191115165132.9472-3-matthew.d.roper@intel.com