gpu: drm: bridge: sii9234: remove unused bridge_to_sii9234 function

clang with W=1 reports
drivers/gpu/drm/bridge/sii9234.c:870:31: error:
  unused function 'bridge_to_sii9234' [-Werror,-Wunused-function]
static inline struct sii9234 *bridge_to_sii9234(struct drm_bridge *bridge)
                              ^
This static function is not used, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230318002321.1675181-1-trix@redhat.com