r600g: Remove streamout-based buffer copy path

r600g: Remove streamout-based buffer copy path

r600g is the only user of util_blitter_copy_buffer in tree, which implements
buffer copies with streamout. This path for r600g was added in 8ac9801669c
("r600g: accelerate buffer copying"), a commit from 2012. At that point there
was no DMA path for buffer copies. Since then, a DMA path has been added,
conditional only on the kernel version -- not the hardware. It appears the
required kernel support has been mainline for at least 4 years now. Mesa 22.2
doesn't need to provide optimal performance on an old kernel -- for performance,
a DMA-capable kernel should be used, and for compatability, the CPU fallback
(used for unaligned buffers as it is) is still available. Remove the streamout
path "in the middle" that appears ~unused today.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17142>