[Intel-gfx] Supporting fused display configurations

[Damien Lespiau]

A while ago, Ben added support for Quanta devices that had the display hardware
fused off. We can a bit more general than that by reading the FUSE_STRAP
register and setting num_pipes to 0 when detecting such condition.

To achieve that, the intel_device_info structure in dev_priv needed to be made
writable, that's patch 1.

Patches 2 to 5 build the support for reading out FUSE_STRAP on IVB+, client
platforms only to detect both "display fused off" and "pipe C fused off"
configurations.

Patch 6 removes the Quanta special-case with the (reasonable) hope that it's
now covered by the DISPLAY_DISABLE bit.

Patch 7 is a bonus, somewhat unrelated, little clean-up patch.

This series is not tested due to lack of such a device, I was hoping Ben still
had the device around somewhere and could give this series a go.