freedreno/a6xx: Document the bit for the magic 32bit-uniforms-as-16b mode.

freedreno/a6xx: Document the bit for the magic 32bit-uniforms-as-16b mode.

Trying to figure out how uniforms were working, I found that computerator
had different behavior from our GL fragment shaders.  Given that 3xx had
an SP_ bit for this (thanks flto@ for the note), it was a matter of
pasting bits of SP_* setup into computerator until I got the GL behavior.
I named it the same as the a3xx register.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6179>