The static assert used in encode deref modes used the fact there was
less than 16 modes that we wanted to compress as an opportunity to reuse
MODE_ENC_GENERIC_BIT as it just happened to represent 16. However if we
add more than 16 modes i.e need to compress to 6 bits not 5 bits then
MODE_ENC_GENERIC_BIT becomes 32 and the logic in the assert breaks.
Instead we more precisely make sure MODE_ENC_GENERIC_BIT is large
enough to fit all but the last 4 generic modes and that the last 4 modes
defined in the enum are in fact the 4 generic modes.
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30654>
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