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 <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720174746.29100-2-seanpaul@chromium.org