drm/vc4: remove unused function

Fix the following clang warning:

drivers/gpu/drm/vc4/vc4_vec.c:201:1: warning: unused function
'to_vc4_vec_connector' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/1618476325-112629-1-git-send-email-jiapeng.chong@linux.alibaba.com