Use explicit break instead of fall-through to break-only case

clang generates a warning if there's no explicit break or fall-through
annotation. The latter would be kind of silly in this case, and not
robust against any future changes turning the fall-through invalid.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10220>

src/amd/compiler/aco_validate.cpp

@@ -585,6 +585,7 @@ bool validate_subdword_operand(chip_class chip, const aco_ptr<Instruction>& inst
    case aco_opcode::global_store_short_d16_hi:
       if (byte == 2 && index == 2)
          return true;
+      break;
    default:
       break;
    }
@@ -663,6 +664,7 @@ unsigned get_subdword_bytes_written(Program *program, const aco_ptr<Instruction>
    case aco_opcode::v_interp_p2_f16:
       if (chip >= GFX9)
          return 2;
+      break;
    default:
       break;
    }