drm/sun4i: Rename some macros in DE2 driver

Now that some knowledge of DE2 is gained, rename or add some macros to
make code more readable.

Max channel macro is removed, since it is not used and it is not clear
if it has right value. Structures in BSP driver shows possibility of 5
channels maximum although there is no SoC with such configuration.

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/20171201060550.10392-9-jernej.skrabec@siol.net