drm/{i915,xe}: move framebuffer bo to parent interface

drm/{i915,xe}: move framebuffer bo to parent interface

Add .framebuffer_init, .framebuffer_fini and .framebuffer_lookup to the
bo parent interface. While they're about framebuffers, they're
specifically about framebuffer objects, so the bo interface is a good
enough fit, and there's no need to add another interface struct.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patch.msgid.link/848d32a44bf844cba3d66e44ba9f20bea4a8352d.1773238670.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>