drm/i915: Update to Linux 4.2

* Work in progress atomic modesetting support

* Broadwell support improvements

* Skylake support improvements, including runtime power management.
Starting from Lin

drm/i915: Update to Linux 4.2

* Work in progress atomic modesetting support

* Broadwell support improvements

* Skylake support improvements, including runtime power management.
Starting from Linux 4.2, a separate firmware blob is required to save
and restore the state of display engines in some low-power modes. These
low-power modes have been forcibly disabled in the DragonFly version of
this driver in order to keep it blob-free.

* Basic Broxton (BXT) support

* Various Displayport improvements including optimized link training,
increased robustness, and automated support for compliance testing

* Execlist overhead reduced

* Frequency boost tuning

* Plenty of bugfixes all around

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