drm/i915: move and group modeset_wq and flip_wq under display.wq

Move display workqueue 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/f34f7fb45510e880ce0cc16cb2fbba72fbe94a1d.1661779055.git.jani.nikula@intel.com