radeonsi: rework polygon stippling to use constant buffer instead of texture

radeonsi: rework polygon stippling to use constant buffer instead of texture

add it to the RW_BUFFERS descriptor array

now the slot masks don't have to have 64 bits

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>