freedreno: Handle non-null cb with null buffer

u_blitter does this to restore constant state, so we need to handle both
cases.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30064>

src/gallium/drivers/freedreno/freedreno_state.c

@@ -152,9 +152,9 @@ fd_set_constant_buffer(struct pipe_context *pctx, enum pipe_shader_type shader,
    util_copy_constant_buffer(&so->cb[index], cb, take_ownership);
 
    /* Note that gallium frontends can unbind constant buffers by
-    * passing NULL here.
+    * passing a NULL cb, or a cb with no buffer:
     */
-   if (unlikely(!cb)) {
+   if (!cb || !(cb->user_buffer || cb->buffer)) {
       so->enabled_mask &= ~(1 << index);
       return;
    }