drm/sun4i: Fix DE2 mixer size

DE2 mixer is always 0x6000 bytes in size on all known SoCs.

While at it, introduce a macro for that.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-11-jernej.skrabec@siol.net