drm: Implement and use the Linux i2c API

* This is mainly to the benefit of the drm/i915 driver

* Sync key portions of code with Linux 4.4

* Key drm functions using the DragonFly-specific iic(4) A

drm: Implement and use the Linux i2c API

* This is mainly to the benefit of the drm/i915 driver

* Sync key portions of code with Linux 4.4

* Key drm functions using the DragonFly-specific iic(4) API
have been copied and renamed with an _iic suffix

* The drm/radeon driver no longer being actively developed, the
intent is to keep it functional with a frozen API

Thanks to Johannes Hofmann for figuring out we could live with
a dummy default bit_xfer() implementation

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