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

To support 4 lanes, re-parent DPI clock source between DSI byteclock
(using the new "variable sources" defined in clk-rp1) and PLL_SYS.
This is to cover cases in which byteclock < pixclock <= 200MHz.

Tidying: All frequencies now in Hz (not kHz), where DSI speed is now
represented by byteclock to simplify arithmetic. Clamp DPI and byte
clocks to their legal ranges; fix up HSTX timeout to avoid an unsafe
assumption that it would return to LP state for every scanline.

Because of RP1's clock topology, the ratio between DSI and DPI clocks
may not be exact with 3 or 4 lanes, leading to slightly irregular
timings each time DSI switches between HS and LP states. Tweak to
inhibit LP during Horizontal BP when sync pulses were requested.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>