drm/doc: Drop misleading comment on drm_mode_config_cleanup

This is not something we'll fix, because failing to clean up stuff (or
doing it in the wrong order) is a driver bug. The offending FIXME goes
all the way back to the original modeset merge.

We've added a WARN_ON in

commit 2b677e8c


Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Dec 10 21:16:05 2012 +0100

    drm: reference framebuffers which are on the idr

including a comment blaming drivers on this. Right thing to do is most
likely drm_atomic_helper_shutdown plus making sure that
drm_mode_config_cleanup is not called too early (i.e. not in driver
unload, but only in the final drm_device release callback).

Cc: Mihail Atanassov <Mihail.Atanassov@arm.com>
Reported-by: Mihail Atanassov <Mihail.Atanassov@arm.com>
Reviewed-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022163717.1064-1-daniel.vetter@ffwll.ch