drm/i915/vbt: add anonymous structs to group DSI VBT defs

The grouping of DSI VBT definitions is hard to follow and match against
the spec. Use anonymous structs and add comments with the spec
description.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/f57ca596aefa3ef0b4ce1f36452410cf745acddd.1754925923.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>