drm/client: Constify modes

drm/client: Constify modes

The modes used by the client code live on the connectors' mode
lists, which are not owned by the client code, and thus it has
no business modifying the modes. Mark the modes const to make
that fact abundantly clear.

v2: Fix up the kunit test

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250303093847.7698-1-ville.syrjala@linux.intel.com