rp1-dsi: Switch to PLL_SYS source for DPI when 8*lanes > bpp

[njhollinghurst]

Replaces #6136 after rebase to rpi-6.6.y and with the changes grouped into what I hope is a sensible order.

This ought in principle to fix the clock rate bug with 4-lane DSI on RP1. It does not break the (single-lane) Raspberry Pi 7" Display.

However, I still cannot get a picture (or even a backlight) out of LT070ME05000.