drm: xlnx: zynqmp_dpsub: Constify mode argument to function

The zynqmp_dp_encoder_mode_set_transfer_unit() function takes a mode
pointer argument that it doesn't need to modify. Make it const.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

drivers/gpu/drm/xlnx/zynqmp_dp.c

@@ -1195,7 +1195,7 @@ static int zynqmp_dp_set_format(struct zynqmp_dp *dp,
  */
 static void
 zynqmp_dp_encoder_mode_set_transfer_unit(struct zynqmp_dp *dp,
-					 struct drm_display_mode *mode)
+					 const struct drm_display_mode *mode)
 {
 	u32 tu = ZYNQMP_DP_MSA_TRANSFER_UNIT_SIZE_TU_SIZE_DEF;
 	u32 bw, vid_kbytes, avg_bytes_per_tu, init_wait;