drm/i915/dp: actually nack test request

... like the comment says. No idea whether this has any effect, but
I guess it's better to not lie to the display by acking a test request
and never following through with it. This goes back to the commit that
originally introduced this code:

commit a60f0e38


Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Oct 20 15:09:17 2011 -0700

    drm/i915: add DP test request handling

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Meh'ed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>