drm/i915: Make for_each_plane() take dev_priv as argument

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>