drm/i915: Nul-terminate legacy debug string

drm/i915: Nul-terminate legacy debug string

Make sure that when we don't have any scheduler attributes for the
request, the string is terminated.

Fixes: 551e8cef7a3f ("drm/i915: Build request info on stack before printk")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517152824.11619-1-chris@chris-wilson.co.uk
(cherry picked from commit 96d4f03c20d04c80026b1ec3643c090cf4f0eb20)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>