nvc0: fix implicit-fallthrough gcc warning

nvc0: fix implicit-fallthrough gcc warning

gcc warning:
../src/gallium/drivers/nouveau/nvc0/nvc0_screen.c: In function ‘nvc0_screen_get_compute_param’:
../src/gallium/drivers/nouveau/nvc0/nvc0_screen.c:623:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
  623 |       switch (obj_class) {
      |       ^~~~~~
../src/gallium/drivers/nouveau/nvc0/nvc0_screen.c:634:4: note: here
  634 |    case PIPE_COMPUTE_CAP_MAX_PRIVATE_SIZE: /* l[] */
      |    ^~~~

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Pierre Moreau <dev@pmoreau.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10544>