st/vega: Match MALLOC/FREE for vg_shader.

A vg_shader is destroyed with FREE.
diff --git a/src/gallium/state_trackers/vega/shaders_cache.c b/src/gallium/state_trackers/vega/shaders_cache.c
index b907a60..53e6bfc 100644
--- a/src/gallium/state_trackers/vega/shaders_cache.c
+++ b/src/gallium/state_trackers/vega/shaders_cache.c
@@ -410,7 +410,7 @@
                                            const char *txt, int num_tokens,
                                            int type)
 {
-   struct vg_shader *shader = (struct vg_shader *)malloc(
+   struct vg_shader *shader = (struct vg_shader *)MALLOC(
       sizeof(struct vg_shader));
    struct tgsi_token *tokens = tokens_from_assembly(txt, num_tokens);
    struct pipe_shader_state state;