Remove plane->pipe mapping from SAREA private after all

Remove plane->pipe mapping from SAREA private after all

We can figure out which pipe a given plane is mapped to by looking at the
display control registers instead of tracking it in a new SAREA private field.
If this becomes a performance problem, we could move to an ioctl based solution
by adding a new parameter for the DDX to set (defaulting to the old behavior if
the param was never set of course).