radeonsi/gfx10: flush gfx cs on ngg -> legacy transition

radeonsi/gfx10: flush gfx cs on ngg -> legacy transition

with a sequence like this:

  glClear(STENCIL)
  glBeginTransformFeedback()
  ...
  glEndTransformFeedback()
  glClear(STENCIL)

The second clear sometimes may produce an unexpected result.

Calling si_flush_gfx_cs() when doing ngg -> legacy transition seems to be a
valid workaround (both for the synthetic reproducer and the real Blender bug).

Using flush flags or events (BOTTOM_OF_PIPE_TS, RESET_TO_LOWEST_VGT) didn't help.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2941
Cc: mesa-stable
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7750>