drm/i915: Constify watermark state checker

The skl+ wm state checker has no reason to modify the crtc state,
so make it const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231004155607.7719-6-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>