drm/damage-helper: add const qualifier in drm_atomic_helper_damage_merged()

Add a const qualifier for the "state" parameter as well as we could
use this helper to get the combined damage in cases of const
drm_plane_state as well. Needed mainly for xe driver big joiner cases
where we need to track the damage from immutable plane state.

Reviewed-by: Ville Syrjälä <[e-mail]>
Signed-off-by: Vinod Govindapillai <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Mika Kahola <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_damage_helper.c

···
  * True if there is valid plane damage otherwise false.
  */
 bool drm_atomic_helper_damage_merged(const struct drm_plane_state *old_state,
-				     struct drm_plane_state *state,
+				     const struct drm_plane_state *state,
 				     struct drm_rect *rect)
 {
 	struct drm_atomic_helper_damage_iter iter;

include/drm/drm_damage_helper.h

···
 drm_atomic_helper_damage_iter_next(struct drm_atomic_helper_damage_iter *iter,
 				   struct drm_rect *rect);
 bool drm_atomic_helper_damage_merged(const struct drm_plane_state *old_state,
-				     struct drm_plane_state *state,
+				     const struct drm_plane_state *state,
 				     struct drm_rect *rect);
 
 #endif