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 <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/drm_modes.c

···
  * @mode: mode
  *
  * Returns:
- * @modes's vrefresh rate in Hz, rounded to the nearest integer. Calculates the
- * value first if it is not yet set.
+ * @modes's vrefresh rate in Hz, rounded to the nearest integer.
  */
 int drm_mode_vrefresh(const struct drm_display_mode *mode)
 {