drm/i915/sdvo: Constify mapping structs

We aren't intending to mutate the SDVO device mapping structs,
so make them const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230829113920.13713-13-ville.syrjala@linux.intel.com


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