drm/tegra: sor: Constify display mode

The data structure is always only read, never written, and can hence be
referred to by a const pointer.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/sor.c

@@ -385,7 +385,7 @@ static int tegra_sor_compute_params(struct tegra_sor *sor,
 }
 
 static int tegra_sor_calc_config(struct tegra_sor *sor,
-				 struct drm_display_mode *mode,
+				 const struct drm_display_mode *mode,
 				 struct tegra_sor_config *config,
 				 struct drm_dp_link *link)
 {