vc4: fix build failure in vc4_dsi_dev_probe()

The bridge->of_node field is defined inside of an #ifdef, which
results in a build failure when compile-testing the vc4_dsi driver
without CONFIG_OF:

drivers/gpu/drm/vc4/vc4_dsi.c: In function 'vc4_dsi_dev_probe':
drivers/gpu/drm/vc4/vc4_dsi.c:1822:20: error: 'struct drm_bridge' has no member named 'of_node'
 1822 |         dsi->bridge.of_node = dev->of_node;

Add another #ifdef in the place it is used in. Alternatively we
could consider dropping the #ifdef in the struct definition
and all other users.

Fixes: 78df6403 ("drm/vc4: dsi: Convert to using a bridge instead of encoder")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230117165258.1979922-1-arnd@kernel.org