radv: Create continue preamble on GFX6 even when no shader rings are used.

radv: Create continue preamble on GFX6 even when no shader rings are used.

Skipping the continue preamble can allow other processes to mess
up some	registers set by the current process.

Originally, we could omit generating the continue preamble when
no shader rings were used, because the register initialization
happened at the beginning of every main cmdbuf. However, this
isn't the case anymore.

Cc: mesa-stable
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22354>