drm/i915: Refactor self-refresh watermark calculations

Move the plane->mode config to the point of use rather than repeatedly
querying the same information.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_drv.h

@@ -202,9 +202,7 @@ struct drm_i915_display_funcs {
 	void (*disable_fbc)(struct drm_device *dev);
 	int (*get_display_clock_speed)(struct drm_device *dev);
 	int (*get_fifo_size)(struct drm_device *dev, int plane);
-	void (*update_wm)(struct drm_device *dev, int planea_clock,
-			  int planeb_clock, int sr_hdisplay, int sr_htotal,
-			  int pixel_size);
+	void (*update_wm)(struct drm_device *dev);
 	/* clock updates for mode set */
 	/* cursor updates */
 	/* render clock increase/decrease */