radeonsi/tmz: add workaround for mpv/vaapi subtitles

radeonsi/tmz: add workaround for mpv/vaapi subtitles

Subtitles are rendering with an upload through a staging texture.
So the sequence is:
  1. draw video (with a secure cs)
  2. copy staging texture to the real texture (via si_resource_copy_region) in
    a non-secure cs.
  3. draw video (with a secure cs)

Step 2 and 3 both generates a flush with RADEON_FLUSH_TOGGLE_SECURE_SUBMISSION.
These flushes are executed quite late: right before doing the draw/dispatch,
so maybe the issue here is the handling of dependencies.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6049>