drm/modes: Fix drm_mode_vrefres() docs

We no longer store a cache vrefresh value in the mode.
Remove the stale information from drm_vrefresh() docs.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241129042629.18280-3-ville.syrjala@linux.intel.com


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