drm/fb-helper: Use drm_fb_helper_restore_fbdev_mode() in drm_fb_helper_set_par()

Use drm_fb_helper_restore_fbdev_mode() in drm_fb_helper_set_par() to
make sure extra planes get disabled whenever fbcon takes over.

Otherwise the code in drm_fb_helper_set_par() was already doing the
exact same thing as drm_fb_helper_restore_fbdev_mode(), so this doesn't
change the behaviour in any other way.

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