drm/vc4: dsi: Rename bridge to out_bridge

In preparation for converting the encoder to being a bridge,
rename the variable holding the next bridge in the chain to
out_bridge, so that our bridge can be called bridge.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20221207-rpi-dsi-bridge-v1-1-8f68ee0b0adb@cerno.tech


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