ARM: dts: stm32: fix display pinmux for stm32f746-disco

As reported by the datasheet (DocID027590 Rev 4) for PG12:
- AF9  -> LCD_B4
- AF14 -> LCD_B1

So replace AF14 with AF9 for PG12 in the dts.

Fixes: fe63d3cf ("ARM: dts: stm32: Sync DT with v4.20 kernel for stm32f7")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>