drm/i915: move fbc to display.fbc

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

Pointers and arrays of pointers to structs that we defined are fine
without a sub-struct wrapping.

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/1151469ec13d392df86b72a375f490fd70a3257a.1661779055.git.jani.nikula@intel.com