drm/doc: Discourage usage of MODESET_CTL ioctl

Leftover from the old days of ums and should be used any longer. Since

commit 29935554
Author: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Date:   Wed May 30 00:58:09 2012 +0200

    drm: Disallow DRM_IOCTL_MODESET_CTL for KMS drivers

it is a complete no-Op for kms drivers.

v2: Fix up mangled sentence spotted by Michel.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Michel Dänzer <michel@daenzer.net>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>