drm/sun4i: Return if frontend is not present

Added return statement in sun4i_layer_format_mod_supported()
in case frontend is not present.

Signed-off-by: Saud Farooqui <farooqui_saud@hotmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/PA4P189MB1421E93EF5F8E8E00E71B7878BB29@PA4P189MB1421.EURP189.PROD.OUTLOOK.COM