drm/i915/execlists: Avoid putting the error pointer

On allocation error, do not jump to the unwind handler that tries to
free the error pointer.

Reported-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: a89d1f92

 ("drm/i915: Split i915_gem_timeline into individual timelines")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180611153332.14824-1-chris@chris-wilson.co.uk