drm: Make some modes const when iterating through them

valid_inferred_mode() don't change the modes over which it iterates,
so make the iterator const.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_edid.c

@@ -2044,7 +2044,7 @@ mode_in_range(const struct drm_display_mode *mode, struct edid *edid,
 static bool valid_inferred_mode(const struct drm_connector *connector,
 				const struct drm_display_mode *mode)
 {
-	struct drm_display_mode *m;
+	const struct drm_display_mode *m;
 	bool ok = false;
 
 	list_for_each_entry(m, &connector->probed_modes, head) {