drm/i915: Sanitize prepare_pipes after valleyview_modeset_global_pipes()

valleyview_modeset_global_pipes() may add pipes that are getting fully
disabled to prepare_pipes bitmask. The rest of the code doesn't expect
this, so clear out any such pipes from the prepare_pipes bitmask.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -9493,10 +9493,14 @@ static int __intel_set_mode(struct drm_crtc *crtc,
 	 * mode set on this crtc.  For other crtcs we need to use the
 	 * adjusted_mode bits in the crtc directly.
 	 */
-	if (IS_VALLEYVIEW(dev))
+	if (IS_VALLEYVIEW(dev)) {
 		valleyview_modeset_global_pipes(dev, &prepare_pipes,
 						modeset_pipes, pipe_config);
 
+		/* may have added more to prepare_pipes than we should */
+		prepare_pipes &= ~disable_pipes;
+	}
+
 	for_each_intel_crtc_masked(dev, disable_pipes, intel_crtc)
 		intel_crtc_disable(&intel_crtc->base);