drm/i915: Make num_sprites a per-pipe value

In the future, we need to be able to specify per-pipe number of
planes/sprites. Let's start today!

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>