drm/client: Extract drm_connector_first_mode()

Use a consistent method for picking the first mode from the
connnector's mode list.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404203336.10454-9-ville.syrjala@linux.intel.com


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