freedreno/a6xx: Cleanup SP_XS_CTRL_REG0 definitions

freedreno/a6xx: Cleanup SP_XS_CTRL_REG0 definitions

The registers were actually different per-stage even though we used the
same type, which resulted in a bunch of incorrectly programmed fields
and confusion. Move the stage-specific values to the registers
themselves, which makes things much less confusing and makes it possible
to set "mergedregs" correctly.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9493>