drm/i915/vbt: split up DSI VBT defs to a separate file

The DSI VBT definitions have ended up in intel_bios.h, because
intel_vbt_defs.h is supposed to be internal to intel_bios.c, but the DSI
VBT definitions are needed in more places.

Split out the DSI VBT definitions to intel_dsi_vbt_defs.h. This will
also help keep the definitions in sync with IGT.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/84417e0141f98ae8f8c7a66e9002c3e99c9ed3db.1754925923.git.jani.nikula@intel.com


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