This is basically just once through the loop but copy and pasted. One
difference is that the single render target case used a headerless
message, and the multiple render target case always used headers.
Now we use headerless messages for the first render target, even in the
multiple render target case. While we already have it set up for the
other RTs, it's still 2 fewer registers to send. Minor improvement.
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20172>
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