drm/fb-helper: Disable cursors and planes when restoring fbdev mode

Cursors and plane can obscure whatever fbdev wants to show the user.
Disable them all in drm_fb_helper_restore_fbdev_mode.

After the cursors and planes have been disabled, user space needs to
explicitly re-enable them to make them visible again.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@gmail.com>