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

drivers/gpu/drm/xlnx/zynqmp_disp.c

@@ -242,12 +242,6 @@ static const u32 scaling_factors_565[] = {
 	ZYNQMP_DISP_AV_BUF_5BIT_SF,
 };
 
-static const u32 scaling_factors_666[] = {
-	ZYNQMP_DISP_AV_BUF_6BIT_SF,
-	ZYNQMP_DISP_AV_BUF_6BIT_SF,
-	ZYNQMP_DISP_AV_BUF_6BIT_SF,
-};
-
 static const u32 scaling_factors_888[] = {
 	ZYNQMP_DISP_AV_BUF_8BIT_SF,
 	ZYNQMP_DISP_AV_BUF_8BIT_SF,