drm/i915: Mark debug mod options as _unsafe

We don't want random people to touch these.

Especially true since we've just screwed up SKL by holding it way too
long under the preliminary flag because of some ABI issues. And now
there's howtos all over the internets about how to set this. Same
pretty much for anything else.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: drop fastboot changes, that option is gone.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>