panfrost: Add an unpacked message preload struct

panfrost: Add an unpacked message preload struct

The compiler will soon produce preloaded messages, but it should not pack them
itself, as this would require depending on GenXML or handcoding bitfields / bit
packs in the compiler. Instead, add a struct encoding the unpacked form of the
message, used as ABI between the compiler and the common driver.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9438>