atrous: Revert to iop_cs_Lab (darktable-org#10798).

This is because it introduces regression on corresponding test:

Test 0024-contrast-equalizer
      Image mire1.cr2
      Expected CPU vs. current CPU report :
      ----------------------------------
      Max dE                   : 32.95917
      Avg dE                   : 4.18790
      Std dE                   : 3.94097
      ----------------------------------
      Pixels below avg + 0 std : 64.59 %
      Pixels below avg + 1 std : 86.84 %
      Pixels below avg + 3 std : 97.71 %
      Pixels below avg + 6 std : 99.98 %
      Pixels below avg + 9 std : 100.00 %
      ----------------------------------
      Pixels above tolerance   : 61.60 %

  FAILS: image visually changed
         see diff.png for visual difference
         (2.72239e+06 pixels changed)

Total test 1
Errors     1
   - 0024-contrast-equalizer
see test-20220118-141420.log

src/iop/atrous.c

@@ -154,7 +154,7 @@ int flags()
 
 int default_colorspace(dt_iop_module_t *self, dt_dev_pixelpipe_t *pipe, dt_dev_pixelpipe_iop_t *piece)
 {
-  return iop_cs_rgb;
+  return iop_cs_Lab;
 }
 
 int legacy_params(dt_iop_module_t *self, const void *const old_params, const int old_version,