drm: atomic helper: do not unreference error pointer

562c5b4d didn't quite fix the issue of dealing with an error
pointer. We can't free/unref an error pointer so reset it to NULL.

Many thanks to Dan Carpenter for pointing this out again.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Daniel Stone <daniels@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: dri-devel@lists.freedesktop.org
Fixes: 562c5b4d ("drm: fix blob pointer check")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1457698646-22231-1-git-send-email-lionel.g.landwerlin@intel.com