drm: make drm_atomic_set_mode_prop_for_crtc() more reliable

drm_atomic_set_mode_prop_for_crtc() does not clear the state->mode, so
old data may be left there when a new mode is set, possibly causing odd
issues.

This patch improves the situation by always clearing the state->mode
first.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_atomic.c

@@ -351,6 +351,8 @@ int drm_atomic_set_mode_prop_for_crtc(struct drm_crtc_state *state,
 	drm_property_unreference_blob(state->mode_blob);
 	state->mode_blob = NULL;
 
+	memset(&state->mode, 0, sizeof(state->mode));
+
 	if (blob) {
 		if (blob->length != sizeof(struct drm_mode_modeinfo) ||
 		    drm_mode_convert_umode(&state->mode,
@@ -363,7 +365,6 @@ int drm_atomic_set_mode_prop_for_crtc(struct drm_crtc_state *state,
 		DRM_DEBUG_ATOMIC("Set [MODE:%s] for CRTC state %p\n",
 				 state->mode.name, state);
 	} else {
-		memset(&state->mode, 0, sizeof(state->mode));
 		state->enable = false;
 		DRM_DEBUG_ATOMIC("Set [NOMODE] for CRTC state %p\n",
 				 state);