drm/i915/bxt: Broxton has 3 sprite planes on pipe A/B, 2 on pipe C

v2: Rebase on top of the for_each_pipe() change adding dev_priv as first
    argument.

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