iris: Emit flushes for push constant source buffers

Push constant loading is not coherent with L3 according to the document
that describes the hardware change for the vertex buffer L3 Bypass
Disable field.

If we've updated a push constant buffer with say, a blorp_buffer_copy,
we may need to flush both the render cache and the tile cache.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Rohan Garg <rohan.garg@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15275>

src/gallium/drivers/iris/iris_state.c

@@ -5617,6 +5617,9 @@ setup_constant_buffers(struct iris_context *ice,
 
       assert(cbuf->buffer_offset % 32 == 0);
 
+      if (res)
+         iris_emit_buffer_barrier_for(batch, res->bo, IRIS_DOMAIN_OTHER_READ);
+
       push_bos->buffers[n].length = range->length;
       push_bos->buffers[n].addr =
          res ? ro_bo(res->bo, range->start * 32 + cbuf->buffer_offset)