drm/i915/skl: Introduce a I915_MAX_PLANES macro

This can be useful to declare structures around pipes and planes and
don't have to go back auditing the code if the next platorm bump that
number.

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