PL110 8-bit mode is not emulated correctly

When the emulated pl110/pl111 is switched programmatically to 8-bit color depth mode, the display is drawn green and blue, but the real PL110 displays grayscale in 8-bit mode.

The bug appears in qemu-system-arm version 3.1.0 (Debian 1:3.1+dfsg-8+deb10u8) and qemu-system-arm version 5.2.50 (v5.2.0-1579-g99ae0cd90d).