ac, aco, radv: Clarify LDS size on GFX6, and NGG shaders.

ac, aco, radv: Clarify LDS size on GFX6, and NGG shaders.

This information was wrong in some places, let's fix it now.

GFX6:
The GPU has 64KB LDS, but only 32KB is usable by a workgroup.

NGG:
There was some misinformation about NGG only being able to
address 32 KB LDS, it turns out this is actually not true
and it can address the full 64K.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21935>