drm/sun4i: Reorganize UI layer code in DE2

Till now, DE2 driver supported only UI planes. Before we add support for
VI planes, lets split out UI layer specific code from common parts. This
commit does the following:
- renames sun8i_layer.c to sun8i_ui_layer.c
- moves UI channel specific code to sun8i_ui_layer.c
- moves common code from sun8i_layer.c to sun8i_mixer.c
- renames function and structure names so it is apparent where they
belong to

No functional change is made.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-19-jernej.skrabec@siol.net