drm: xlnx: remove defined but not used 'scaling_factors_666'

This addresses the following gcc warning with "make W=1":

drivers/gpu/drm/xlnx/zynqmp_disp.c:245:18: warning:
‘scaling_factors_666’ defined but not used [-Wunused-const-variable=]
  245 | static const u32 scaling_factors_666[] = {
      |                  ^~~~~~~~~~~~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200910140630.1191782-1-yanaijie@huawei.com