st/mesa: Don't set atomic counter size != 0 if MAX_SHADER_BUFFERS == 0.

st/mesa: Don't set atomic counter size != 0 if MAX_SHADER_BUFFERS == 0.

This is just asking for tests to get confused about the HW supporting
atomics in this shader stage or not, such as
dEQP-GLES31.functional.shaders.opaque_type_indexing.atomic_counter.const_expression_vertex.

v2: Rebase on the other atomic cleanups that have happened since posting.
v3: Commit message tweak by Marek.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>