applet.display.pdi: modify to use port groups.

[q3k]

Another one for #599, this time a slightly more complex case:

1. The applet uses applet.interface.spi_controller.SPIController{Subtarget,Interface} which stays at the old pads API;
2. __pins were used to call access.add_pin_argument - this loop has been unrolled, as I assume removing __pins takes precedence?

[whitequark]

LGTM. Yeah, I'd say unroll the loop, the __pins pattern is really weird.

Eventually there's going to be another major migration that changes how applets are composed, but it'll heavily build on the get_port_group() style infrastructure so it'll be less disruptive.

After that migration we'll have ports declared in a way kinda like...

class AppletInterface:
    sck: PortSpec.Input
    io: PortSpec.Bidir[4]

or something like that (syntax extremely TBD). There, superapplets could route some of their pins to subapplets.

But this is all extremely up in the air, and the migration I requested takes priority at the moment, as the Pads stuff that dates back to Migen really must go.