drm/i915: Add a for_each_sprite() macro

This macro is similar to for_each_pipe() we already have. Convert the
two call sites we have at the same time.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>