drm/sun4i: layer: Add tiled modifier support and helper

This introduces a list of supported modifiers for the driver, that
includes the Allwinner tiled modifier, as well as a format_mod_supported
callback.

The callback uses both the backend and frontend helpers to indicate
per-format modifier support (including for the linear modifier).

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190118145133.21281-16-paul.kocialkowski@bootlin.com