drm/i915: adjust sprite base address

Just like in:

commit c2c75131
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 5 12:17:30 2012 +0200

    drm/i915: adjust framebuffer base address on gen4+

but this time, for the sprite planes. This ensures that the
sprite offset are always inside the supported hardware limits since it
becomes the offset into a page and we adjust the base address to a page
boundary.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>