drm/sun4i: Implement zpos for DE2

Initial implementation of DE2 planes only supported fixed zpos.

Expand implementation with configurable zpos property.

Implementation background:
Channel in DE2 driver represents one DRM plane, whereas pipe is just
mapped channel to known Z position. Pipe 0 will always be at the bottom,
pipe 1 just above pipe 0 and so on. If, for example, channel 1 is mapped
at pipe 0 and channel 0 at pipe 1, whatever is on channel 0 will appear
on top.

Before this commit, channel id was used for addressing channel related
registers (prefixed with SUN8I_MIXER_CHAN_UI_ or SUN8I_MIXER_CHAN_VI_)
and pipe registers (prefixed with SUN8I_MIXER_BLEND_). Additionally,
register SUN8I_MIXER_BLEND_ROUTE, which takes care for mapping channels
to pipes had fixed value. It mapped channel 0 to pipe 0, 1 to 1 and so
on. Consequence of all that was fixed Z order of planes.

With this commit, pipe registers are using zpos property as index and
channel related registers still use channel id as index. Pipe mapping
register is now set dynamically too and pipe enable register is rebuild
every time to make sure only active pipes are enabled.

Testing was done to confirm that there is no issues if bottom plane
contains pixels with alpha value < 0xff and if it doesn't whole screen.

Tested-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
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/20180706164732.24166-1-jernej.skrabec@siol.net