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: 247870ac ("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