drm/i915: fix up the relocate_entry refactoring

Somehow we've lost the error handling in the patch split-up between
the internal and external patch. This regression has been introduced
in

commit 5032d871
Author: Rafael Barbalho <rafael.barbalho@intel.com>
Date:   Wed Aug 21 17:10:51 2013 +0100

    drm/i915: Cleaning up the relocate entry function

This bug is exercised by igt/gem_reloc_vs_gpu/interruptible.

Cc: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>