drm/sun4i: Pass modifier to backend and frontend format support helpers

To prepare the introduction of tiled mode support, pass the framebuffer
format modifier to the helpers dealing with format support.

Since only linear mode is supported for now, add corresponding checks in
each helper.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-33-paul.kocialkowski@bootlin.com