drm/nouveau/kms/nv50: remove set but not unused variable 'nv_connector'

drivers/gpu/drm/nouveau/dispnv50/disp.c: In function nv50_pior_enable:
drivers/gpu/drm/nouveau/dispnv50/disp.c:1672:28: warning:
 variable nv_connector set but not used [-Wunused-but-set-variable]

commit ac2d9275 ("drm/nouveau/kms/nv50-: Store the
bpc we're using in nv50_head_atom") left behind this.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>