Somehow UXA submits a completely bogus DR4 value since essentially
forever. It was originally introduced in

commit bade7d7d2505a10a8a7d24b084aff9742e2d6d64
Author: Eric Anholt <eric@anholt.net>
Date:   Fri Jun 6 14:03:25 2008 -0700

    Use the DRM for submitting batchbuffers when available.

and dutifully copied around ever since. Since we want to keep the
general dirt catching around just special case the UXA value.

This regression was introduced in

commit 9cb34664
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Apr 24 08:09:11 2014 +0200

    drm/i915: Catch dirt in unused execbuffer fields

Comment from Chris' review:

"To be fair, it is a sensible value if one supposes a Region style API to
cliprects. Under that API, DR[14] define the extents of the clip region,
and ((0,0), (0,0)) [DR1==DR4==0] would mean all clipped, do not draw
anything."

v2: Pimp commit message a bit and remove the double space.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78494
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jörg Otte <jrg.otte@gmail.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>