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: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -76,6 +76,14 @@ enum transcoder {
 };
 #define transcoder_name(t) ((t) + 'A')
 
+/*
+ * This is the maximum (across all platforms) number of planes (primary +
+ * sprites) that can be active at the same time on one pipe.
+ *
+ * This value doesn't count the cursor plane.
+ */
+#define I915_MAX_PLANES	3
+
 enum plane {
 	PLANE_A = 0,
 	PLANE_B,