drm/rockchip: Fix return type of cdn_dp_connector_mode_valid

The mode_valid field in drm_connector_helper_funcs is expected to be of
type:
enum drm_mode_status (* mode_valid) (struct drm_connector *connector,
				     struct drm_display_mode *mode);

The mismatched return type breaks forward edge kCFI since the underlying
function definition does not match the function hook definition.

The return type of cdn_dp_connector_mode_valid should be changed from
int to enum drm_mode_status.
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1703
Cc: llvm@lists.linux.dev
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913205555.155149-1-nhuck@google.com

drivers/gpu/drm/rockchip/cdn-dp-core.c

@@ -283,7 +283,8 @@ static int cdn_dp_connector_get_modes(struct drm_connector *connector)
 	return ret;
 }
 
-static int cdn_dp_connector_mode_valid(struct drm_connector *connector,
+static enum drm_mode_status
+cdn_dp_connector_mode_valid(struct drm_connector *connector,
 			    struct drm_display_mode *mode)
 {
 	struct cdn_dp_device *dp = connector_to_dp(connector);