drm/nouveau/disp: remove unused function in sorg94.c

We get 1 warning when building kernel with W=1:
drivers/gpu/drm/nouveau/nvkm/engine/disp/sorg94.c:49:1: warning: no previous prototype for 'g94_sor_output_new' [-Wmissing-prototypes]

In fact, this function is called by no one and not exported,
so this patch removes it.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>