drm/i915: Use a pipe variable to cycle through the pipes

I recently fumbled a patch because I wrote twice num_sprites[i], and it
was the right thing to do in only 50% of the cases.

This patch ensures I need to write num_sprites[pipe], ie it should be
self-documented that it's per-pipe number of sprites without having to
look at what is 'i' this time around.

It's all a lame excuse, but it does make it harder to redo the same
mistake.

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>