hw/display/tcx: Add missing fall through comments

hw/display/tcx: Add missing fall through comments

When building with GCC9 using CFLAG -Wimplicit-fallthrough=2 we get:

  hw/display/tcx.c: In function ‘tcx_dac_writel’:
  hw/display/tcx.c:453:26: error: this statement may fall through [-Werror=implicit-fallthrough=]
    453 |             s->dac_index = (s->dac_index + 1) & 0xff; /* Index autoincrement */
        |             ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
  hw/display/tcx.c:454:9: note: here
    454 |         default:
        |         ^~~~~~~
  hw/display/tcx.c: In function ‘tcx_dac_readl’:
  hw/display/tcx.c:412:22: error: this statement may fall through [-Werror=implicit-fallthrough=]
    412 |         s->dac_index = (s->dac_index + 1) & 0xff; /* Index autoincrement */
        |         ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
  hw/display/tcx.c:413:5: note: here
    413 |     default:
        |     ^~~~~~~
  cc1: all warnings being treated as errors

Give a hint to GCC by adding the missing fall through comments.

Fixes: 55d7bfe22
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20191218192526.13845-3-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>