fix "Missing break in switch" coverity reports

Many of these are marked as "intentional/fix required" because they
just need adding a fall through comment.  This is exactly what this
patch does, except for target/mips/translate.c where it is easier to
duplicate the code, and hw/audio/sb16.c where I consulted the DOSBox
sources and decide to just remove the LOG_UNIMP before the fallthrough.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/display/cg3.c

@@ -232,6 +232,7 @@ static void cg3_reg_write(void *opaque, hwaddr addr, uint64_t val,
                 s->b[s->dac_index] = regval;
                 /* Index autoincrement */
                 s->dac_index = (s->dac_index + 1) & 0xff;
+                /* fall through */
             default:
                 s->dac_state = 0;
                 break;