radv: use a define for the transition point between cp and compute shader

radv: use a define for the transition point between cp and compute shader

For certain buffer meta ops we can use the CP or a compute shader,
we should use a define to rather than hardcoding 4096, allows
for easier testing and more consistency.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>