amd: massively simplify how info->spi_cu_en is applied

amd: massively simplify how info->spi_cu_en is applied

Instead of having ac_set_reg_cu_en that sets the register, replace it with
ac_apply_cu_en that only returns the modified register value,
which allows a large simplification in both drivers because a lot of code
becomes duplicated after it's switched to ac_apply_cu_en.

RADV also didn't apply it to a few registers. Fixed.

This removes 82 lines of code in total.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21641>