drm/sun4i: frontend: Rework a bit the phase data

The scaler filter phase setup in the allwinner kernel has two different
cases for setting up the scaler filter, the first one using different phase
parameters for the two channels, and the second one reusing the first
channel parameters on the second channel.

The allwinner kernel has a third option where the horizontal phase of the
second channel will be set to a different value than the vertical one (and
seems like it's the same value than one used on the first channel).
However, that code path seems to never be taken, so we can ignore it for
now, and it's essentially what we're doing so far as well.

Since we will have always the same values across each components of the
filter setup for a given channel, we can simplify a bit our frontend
structure by only storing the phase value we want to apply to a given
channel.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20201015093642.261440-1-maxime@cerno.tech