drm: rcar-du: Use LVDS PLL clock as dot clock when possible

drm: rcar-du: Use LVDS PLL clock as dot clock when possible

On selected SoCs, the DU can use the clock output by the LVDS encoder
PLL as its input dot clock. This feature is optional, but on the D3 and
E3 SoC it is often the only way to obtain a precise dot clock frequency,
as the other available clocks (CPG-generated clock and external clock)
usually have fixed rates.

Add a DU model information field to describe which DU channels can use
the LVDS PLL output clock as their input clock, and configure clock
routing accordingly.

This feature is available on H2, M2-W, M2-N, D3 and E3 SoCs, with D3 and
E3 being the primary targets. It is left disabled in this commit, and
will be enabled per-SoC after careful testing.

At the hardware level, clock routing is configured at runtime in two
steps, first selecting an internal dot clock between the LVDS PLL clock
and the external DOTCLKIN clock, and then selecting between the internal
dot clock and the CPG-generated clock. The first part requires stopping
the whole DU group in order for the change to take effect, thus causing
flickering on the screen. For this reason we currently hardcode the
clock source to the LVDS PLL clock if available, and allow flicker-free
selection of the external DOTCLKIN clock or CPG-generated clock
otherwise. A more dynamic clock selection process can be implemented
later if the need arises.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>