drm/vc4: dsi: Introduce a variant structure

Most of the differences between DSI0 and DSI1 are handled through the
ID. However, the BCM2711 DSI is going to introduce one more variable to
the mix and will break some expectations of the earlier, simpler, test.

Let's add a variant structure that will address most of the differences
between those three controllers.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201203132543.861591-5-maxime@cerno.tech