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

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