drm/i915/skl: Fix up positive error code

It should have been negative since it is returned with ERR_PTR().

Introduced in new code commit:

   commit 50470bb0
   Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
   Date:   Mon Mar 23 11:10:36 2015 +0000

    drm/i915/skl: Support secondary (rotated) frame buffer mapping

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>