st/mesa: merge 3 unlikely blocks in _mesa_get_bufferobj_reference

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27587>

src/mesa/main/bufferobj.h

@@ -44,26 +44,32 @@ _mesa_get_bufferobj_reference(struct gl_context *ctx, struct gl_buffer_object *o
 
    struct pipe_resource *buffer = obj->buffer;
 
-   if (unlikely(!buffer))
-      return NULL;
-
    /* Only one context is using the fast path. All other contexts must use
     * the slow path.
     */
-   if (unlikely(obj->private_refcount_ctx != ctx)) {
-      p_atomic_inc(&buffer->reference.count);
+   if (unlikely(obj->private_refcount_ctx != ctx ||
+                obj->private_refcount <= 0)) {
+      if (buffer) {
+         if (obj->private_refcount_ctx != ctx) {
+            p_atomic_inc(&buffer->reference.count);
+         } else {
+            /* This is the number of atomic increments we will skip. */
+            const unsigned count = 100000000;
+            p_atomic_add(&buffer->reference.count, count);
+
+            /* Remove the reference that we return. */
+            assert(obj->private_refcount == 0);
+            obj->private_refcount = count - 1;
+         }
+      }
       return buffer;
    }
 
-   if (unlikely(obj->private_refcount <= 0)) {
-      assert(obj->private_refcount == 0);
-
-      /* This is the number of atomic increments we will skip. */
-      obj->private_refcount = 100000000;
-      p_atomic_add(&buffer->reference.count, obj->private_refcount);
-   }
-
-   /* Return a buffer reference while decrementing the private refcount. */
+   /* Return a buffer reference while decrementing the private refcount.
+    * The buffer must be non-NULL, which is implied by private_refcount_ctx
+    * being non-NULL.
+    */
+   assert(buffer);
    obj->private_refcount--;
    return buffer;
 }