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>

src/mesa/state_tracker/st_extensions.c

@@ -259,7 +259,7 @@ void st_init_limits(struct pipe_screen *screen,
           */
          pc->MaxAtomicCounters = temp;
          pc->MaxAtomicBuffers = screen->get_shader_param(screen, sh, PIPE_SHADER_CAP_MAX_HW_ATOMIC_COUNTER_BUFFERS);
-      } else {
+      } else if (pc->MaxShaderStorageBlocks) {
          pc->MaxAtomicCounters = MAX_ATOMIC_COUNTERS;
          /*
           * without separate atomic counters, reserve half of the available