drm/sun4i: de2: Don't return de2_fmt_info struct

Now that de2_fmt_info contains only DRM <-> HW format mapping, it
doesn't make sense to return pointer to structure when searching by DRM
format. Rework that to return only HW format instead.

This doesn't make any functional change.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224173901.174016-7-jernej.skrabec@siol.net