sms/vdp: fix bad mirroring

On the real machine, I ended up at some point getting a weird "bottom
mirroring" issue. It turns out that it was because when I revamped my
VDP flags in an earlier commit, I hadn't read the whole technical
notes about register $02 and falsely thought that B0 was ignored. No
it's not. When set, it mirrors the bottom 8 rows.

arch/z80/sms/blk/605

@@ -1,7 +1,7 @@
 CREATE _idat
 0b00000100 C, 0x80 C, ( Bit 2: Select mode 4 )
 0b00000000 C, 0x81 C,
-0b00001110 C, 0x82 C, ( Name table: 0x3800 )
+0b00001111 C, 0x82 C, ( Name table: 0x3800, *B0 must be 1* )
 0b11111111 C, 0x85 C, ( Sprite table: 0x3f00 )
 0b11111111 C, 0x86 C, ( sprite use tiles from 0x2000 )
 0b11111111 C, 0x87 C, ( Border uses palette 0xf )