Implicit usage of local variables in macros isn't exactly the greatest
thing in the world, especially when that variable is the drm device and
we want to move towards a broader use of the i915 device structure.

Let's make for_each_plane() take dev_priv as its first argument then.
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>