radeonsi: don't flush CS before and after every blitter invocation

u_blitter always calls set_stream_output_targets before and after clears
and blits, which always flushes here. Strengthen the flushing condition
and add an early exit.

No clue why this fixes the test.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27059>

src/gallium/drivers/radeonsi/ci/gfx11-gfx1100-fail.csv

@@ -151,5 +151,4 @@ KHR-GL46.transform_feedback_overflow_query_ARB.multiple-streams-multiple-buffers
 KHR-GL46.transform_feedback_overflow_query_ARB.multiple-streams-one-buffer-per-stream,Fail
 
 # I saw these on Navi31, but not Navi33
-dEQP-GLES3.functional.fragment_ops.interaction.basic_shader.39,Fail
 dEQP-GLES3.functional.fragment_ops.interaction.basic_shader.94,Fail

src/gallium/drivers/radeonsi/si_state_streamout.c

@@ -64,6 +64,9 @@ static void si_set_streamout_targets(struct pipe_context *ctx, unsigned num_targ
    unsigned old_num_targets = sctx->streamout.num_targets;
    unsigned i;
 
+   if (!old_num_targets && !num_targets)
+      return;
+
    /* We are going to unbind the buffers. Mark which caches need to be flushed. */
    if (old_num_targets && sctx->streamout.begin_emitted) {
       /* Stop streamout. */
@@ -102,7 +105,7 @@ static void si_set_streamout_targets(struct pipe_context *ctx, unsigned num_targ
     *    spec@ext_transform_feedback@immediate-reuse-index-buffer
     *    spec@ext_transform_feedback@immediate-reuse-uniform-buffer
     */
-   if (sctx->gfx_level >= GFX11)
+   if (sctx->gfx_level >= GFX11 && old_num_targets)
       si_flush_gfx_cs(sctx, 0, NULL);
 
    /* Streamout buffers must be bound in 2 places: