drm/sysfb: Do not dereference NULL pointer in plane reset

The plane state in __drm_gem_reset_shadow_plane() can be NULL. Do not
deref that pointer, but forward NULL to the other plane-reset helpers.
Clears plane->state to NULL.

v2:
- fix typo in commit description (Javier)

Signed-off-by: Thomas Zimmermann <[email protected]>
Fixes: b715650 ("drm/gem: Export implementation of shadow-plane helpers")
Reported-by: Dan Carpenter <[email protected]>
Closes: https://lore.kernel.org/dri-devel/[email protected]/
Cc: Thomas Zimmermann <[email protected]>
Cc: Melissa Wen <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Simona Vetter <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v5.15+
Reviewed-by: Javier Martinez Canillas <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: tdz

drivers/gpu/drm/drm_gem_atomic_helper.c

@@ -310,8 +310,12 @@ EXPORT_SYMBOL(drm_gem_destroy_shadow_plane_state);
 void __drm_gem_reset_shadow_plane(struct drm_plane *plane,
 				  struct drm_shadow_plane_state *shadow_plane_state)
 {
-	__drm_atomic_helper_plane_reset(plane, &shadow_plane_state->base);
-	drm_format_conv_state_init(&shadow_plane_state->fmtcnv_state);
+	if (shadow_plane_state) {
+		__drm_atomic_helper_plane_reset(plane, &shadow_plane_state->base);
+		drm_format_conv_state_init(&shadow_plane_state->fmtcnv_state);
+	} else {
+		__drm_atomic_helper_plane_reset(plane, NULL);
+	}
 }
 EXPORT_SYMBOL(__drm_gem_reset_shadow_plane);