drm/i915: Apply alignment restrictions on scanout surfaces for VT-d

From the w/a database:

'To prevent false VT-d type 6 error:

  The primary display plane must be 256KiB aligned, and require an extra
  128 PTEs of padding afterward;

  The sprites planes must be 128KiB aligned, and require an extra 64 PTEs
  of padding afterward;

  The cursors must be 64KiB aligned, and require an extra 2 PTEs of
  padding afterward.'

As we use the same function to pin the primary and sprite planes, we can
simply use the more strict requirements for scanouts for both.

Instead of using explicit padding PTEs following the scanout objects, we
should be able to use the scratch page that is always mapped into the
unused PTEs to avoid the VT-d error.

References: https://bugs.freedesktop.org/show_bug.cgi?id=59626
References: https://bugs.freedesktop.org/show_bug.cgi?id=59627
References: https://bugs.freedesktop.org/show_bug.cgi?id=59631


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Apply s/vtd_wa/vtd_scanout_wa/ bikeshed since Damien likes
it, too.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>