Fix morph target motion blur (#23894)

I didn't update a call to `morph_prev_vertex` in commit
dbb003c
(part of #23023), so the prepass shader fails to compile if motion blur
is enabled.

## Testing

```sh
cargo run --example many_morph_targets -- --count 4 --motion-blur
cargo run --example many_morph_targets -- --count 4
```

Author: greeble-dev

crates/bevy_pbr/src/prepass/prepass.wgsl

@@ -140,7 +140,7 @@ fn vertex(vertex_no_morph: Vertex) -> VertexOutput {
 #ifdef MORPH_TARGETS
 
 #ifdef HAS_PREVIOUS_MORPH
-    let prev_vertex = morph_prev_vertex(vertex_no_morph);
+    let prev_vertex = morph_prev_vertex(vertex_no_morph, vertex_no_morph.instance_index);
 #else   // HAS_PREVIOUS_MORPH
     let prev_vertex = vertex_no_morph;
 #endif  // HAS_PREVIOUS_MORPH