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 <ville.syrjala@linux.intel.com>
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250228093802.27091-3-vinod.govindapillai@intel.com

drivers/gpu/drm/drm_damage_helper.c

@@ -308,7 +308,7 @@ EXPORT_SYMBOL(drm_atomic_helper_damage_iter_next);
  * 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

@@ -78,7 +78,7 @@ bool
 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