radeonsi: delay sample_pos_buffer creation until first use

And use pipe_buffer_create_with_data instead of doing it
manually.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11296>

src/gallium/drivers/radeonsi/si_pipe.c

@@ -717,11 +717,6 @@ static struct pipe_context *si_create_context(struct pipe_screen *screen, unsign
    if (!sctx->dirty_implicit_resources)
       goto fail;
 
-   sctx->sample_pos_buffer =
-      pipe_buffer_create(sctx->b.screen, 0, PIPE_USAGE_DEFAULT, sizeof(sctx->sample_positions));
-   pipe_buffer_write(&sctx->b, sctx->sample_pos_buffer, 0, sizeof(sctx->sample_positions),
-                     &sctx->sample_positions);
-
    /* The remainder of this function initializes the gfx CS and must be last. */
    assert(sctx->gfx_cs.current.cdw == 0);
 

src/gallium/drivers/radeonsi/si_state.c

@@ -2947,6 +2947,11 @@ static void si_set_framebuffer_state(struct pipe_context *ctx,
       si_mark_atom_dirty(sctx, &sctx->atoms.s.msaa_config);
       si_mark_atom_dirty(sctx, &sctx->atoms.s.db_render_state);
 
+      if (!sctx->sample_pos_buffer) {
+         sctx->sample_pos_buffer = pipe_buffer_create_with_data(&sctx->b, 0, PIPE_USAGE_DEFAULT,
+                                                      sizeof(sctx->sample_positions),
+                                                      &sctx->sample_positions);
+      }
       constbuf.buffer = sctx->sample_pos_buffer;
 
       /* Set sample locations as fragment shader constants. */