drm/panthor: Fix sync-only jobs

A sync-only job is meant to provide a synchronization point on a
queue, so we can't return a NULL fence there, we have to add a signal
operation to the command stream which executes after all other
previously submitted jobs are done.

v2:
- Fixed a UAF bug
- Added R-bs

Fixes: de85488138 ("drm/panthor: Add the scheduler logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240703071640.231278-3-boris.brezillon@collabora.com

include/uapi/drm/panthor_drm.h

@@ -802,6 +802,9 @@ struct drm_panthor_queue_submit {
 	 * Must be 64-bit/8-byte aligned (the size of a CS instruction)
 	 *
 	 * Can be zero if stream_addr is zero too.
+	 *
+	 * When the stream size is zero, the queue submit serves as a
+	 * synchronization point.
 	 */
 	__u32 stream_size;
 
@@ -822,6 +825,8 @@ struct drm_panthor_queue_submit {
 	 * ensure the GPU doesn't get garbage when reading the indirect command
 	 * stream buffers. If you want the cache flush to happen
 	 * unconditionally, pass a zero here.
+	 *
+	 * Ignored when stream_size is zero.
 	 */
 	__u32 latest_flush;