drm/i915/bdw: Return -ENONENT on default ctx destroy

This was an accidental "ABI" change introduced during PPGTT:

commit 0eea67eb
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:19 2013 -0800

    drm/i915: Create a per file_priv default context

The failure test application actually tests the return type. The other
option is to simply change the test.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>