drm/i915: Avoid format string expansion from engine names

While highly unlikely, this makes sure that the string built from
engine names won't be processed as a format string.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170411045630.GA6612@beast