Raspberry Pi KMS-only driver

Here's the first patch series for graphics on the Raspberry Pi. It brings up fbcon on KMS, and the xf86-video-modesetting driver runs so you can do X with no further userspace drivers.

Right now this series doesn't support changing video modes away from the firmware boot setup, because when I set the pixel clock using the rpi-dt-clocks series I end up with a black display. even on no-op modesets.

I've dropped all of the 3D bits from this series, to simplify this review stage. See vc4-kms-v3d-rpi2 for a series that works with the current Mesa driver, based on downstream 2709. I squashed all of my messy KMS series down, but kept Derek's patches separate to make sure he gets credit.

This code can also be found at:

https://github.com/anholt/linux/tree/vc4-kms-squash