radv/winsys: set IB flags prior to submit in the sysmem path

radv/winsys: set IB flags prior to submit in the sysmem path

This fixes missing scene objects in ZUSI 3 + dxvk. Index / vertex buffer
upload using thousands of CopyBuffer commands in one huge Vulkan command
buffer, mixed with lots of render pass begin/end and draw calls, failed
for some of the buffers.

radv divides the huge command buffer into 3 IBs, and they had random
flags set because the field was uninitialized. Maybe IBs got discarded
if they had the PREAMBLE bit set.

Signed-off-by: Florian Will <florian.will@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: <mesa-stable@lists.freedesktop.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3577>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3577>