drm/rockchip: Fix return type of cdn_dp_connector_mode_valid

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 <[email protected]>
Link: https://github.com/ClangBuiltLinux/linux/issues/1703
Cc: [email protected]
Signed-off-by: Nathan Huckleberry <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Signed-off-by: Heiko Stuebner <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]