drm/i915: move vbt to display.vbt

Move display VBT related members under drm_i915_private display
sub-struct.

v2: Rebase

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/db4b648b201ea0b79654fec2028120999a735db0.1661779055.git.jani.nikula@intel.com