drm/i915: move and group max_bw and bw_obj under display.bw

Move display bandwidth 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/c8b9e2fdc5c226ffb71759a20e561c832a774ba5.1661779055.git.jani.nikula@intel.com