drm/sun4i: Sort includes in VI and UI layer code

sun8i_mixer.h include is misplaced. Move it.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224173901.174016-8-jernej.skrabec@siol.net