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: 247870ac8ea7 ("drm/i915: Build request info on stack before printk")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit 96d4f03c20d04c80026b1ec3643c090cf4f0eb20)
Signed-off-by: Jani Nikula <[email protected]>