drm/vc4: dsi: Move initialisation to encoder_mode_set

Breaking the bridge chain does not work for atomic bridges/panels
and generally causes issues.
We need to initialise the DSI host before the bridge pre_enables
are called, so move that to encoder_mode_set in the same way that
dw-mipi-dsi does.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20221207-rpi-dsi-bridge-v1-2-8f68ee0b0adb@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>