radv,aco,ac/llvm: fix indirect dispatches on the compute queue on GFX7-10

radv,aco,ac/llvm: fix indirect dispatches on the compute queue on GFX7-10

Since neither PKT3_LOAD_SH_REG_INDEX nor PKT3_COPY_DATA work with compute
queues on GFX7-10, we have to load the dispatch size from memory in the
shader.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15064>