drm/i915: move dbuf under display sub-struct

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

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/3363a516c12bd8bfb240131e9eb9fc3a0f3057a3.1661779055.git.jani.nikula@intel.com