drm/i915: move and group fbdev under display.fbdev

Move display fbdev 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>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7eb563bc88623bd3f12b9a00efec85e6a78d5800.1661346845.git.jani.nikula@intel.com