When checking the readport config for a number of sources that
would be used in a multi-slot instruction, then return the index
of the source for which readport allocation fails.
With that we can later add a mov instruction to change the source
channels configuration.
In addition pass a std::array instead of a C-array to make the interface
more c++-like and prepare for the next patch.
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36860>
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