drm/i915/bios: add DP max link rate to VBT child device struct

Update VBT defs to reflect revision 216. While at it, default the
expected child device struct size to sizeof the size rather than a
hardcoded value.

v2: Fix bit order (David)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180118153310.32437-1-jani.nikula@intel.com