drm/modes: Fix drm_mode_is_420_only() comment

Fixes the following warnings when building docs:
../drivers/gpu/drm/drm_modes.c:1623: warning: No description found for parameter 'display'
../drivers/gpu/drm/drm_modes.c:1623: warning: Excess function parameter 'connector' description in 'drm_mode_is_420_only'

Signed-off-by: Sean Paul <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_modes.c

···
  * drm_mode_is_420_only - if a given videomode can be only supported in YCBCR420
  * output format
  *
- * @connector: drm connector under action.
+ * @display: display under action
  * @mode: video mode to be tested.
  *
  * Returns: