etnaviv: don't try to copy PIPE_BUFFER with the 3D engine

etnaviv: don't try to copy PIPE_BUFFER with the 3D engine

PIPE_BUFFER layout is incompatible with the 3D pipe, so don't try to
blit it via a 3D engine blit, but fall back to the software copy.

Fixes crashes in piglit tests arb_copy_buffer and arb_map_buffer_range.

Fixes: c9e8b49b8852 (etnaviv: gallium driver for Vivante GPUs)
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9310>