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
(cherry picked from commit c4fb60b9aba9f939d3f8575df23fd8d5958ec6ed)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>