drm/i915/frontbuffer: remove obsolete comment about mark busy/idle

This no longer exists.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606122203.13416-4-jani.nikula@intel.com