drm/sun4i: Cleanup video/YUV source before enabling a layer

This adds a dedicated function for cleaning the video and YUV source
channel layer enable bits. This function is called first on layer atomic
update to make sure that there are no leftover bits from previous
plane configuration that were not cleaned until now.

It fixes issues when alternating between video and YUV planes, where
both bits would be set eventually, leading to broken plane display.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-2-paul.kocialkowski@bootlin.com