drm/i915: Update to Linux 4.1

drm/i915: Update to Linux 4.1

* Valleyview support has been vastly improved and is no longer considered
  preliminary

* Skylake support improvements: runtime power management, turbo and sleep
  states should now be fully operational.
  Many workarounds have been added for Skylake specific issues

* Preliminary changes to prepare for Broxton (future Atom SOCs) support

* Distinguish hardware minimum and user minimum frequencies. Set the GPU
  frequency to the hardware minimum on idle in order to reduce power usage

* DRRS (dynamic refresh rate switching) is now enabled where supported.
  The idea is to reduce the refresh rate of the panel to save power when
  nothing changes on the screen

* DP deadlock bugfixes and improved link rate computation. Intermediate
  link rate support for eDP 1.4

* XenGT client-side support. This is paravirtualization to allow virtual
  machines to tap into the render engines

* Plenty of internal work to prepare for atomic mode setting

* Lots of other smaller work all over such as added documentation, dead
  UMS code removal, vblank interrupt cleanings, etc...