linux-firmware: Install i915 firmware files

From *Intel Graphics Firmware* [1]:

> ### Description ###
>
> New generations of Intel® Graphics for Linux requires below Firmware
> components to be integreated in KMD.
>
> ### GuC ###
>
> GuC is designed to perform graphics workload scheduling on the various
> graphics parallel engines. In this scheduling model, host software
> submits work through one of the 256 graphics doorbells and this
> invokes the scheduling operation on the appropriate graphics engine.
> Scheduling operations include determining which workload to run next,
> submitting a workload to a command streamer, pre-empting existing
> workloads running on an engine, monitoring progress and notifying host
> SW when work is done.
>
> ### DMC ###
>
> DMC provides additional graphics low-power idle states. It provides
> capability to save and restore display registers across these
> low-power states independently from the OS/Kernel.

This addresses the Linux warnings below on an Intel Sky Lake system.

```
$ uname -a
Linux keineahnung.molgen.mpg.de 4.8.0-rc7.mx64.104 #1 SMP Mon Sep 19 14:46:16 CEST 2016 x86_64 GNU/Linux
$ lspci -nn -s 0:02.0
00:02.0 VGA compatible controller [0300]: Intel Corporation HD Graphics 530 [8086:1912] (rev 06)
$ dmesg | grep i915
[    2.459587] snd_hda_intel 0000:00:1f.3: failed to add i915 component master (-19)
[    2.482612] snd_hda_codec_hdmi hdaudioC0D2: No i915 binding for Intel HDMI/DP codec
[   15.109279] i915 0000:00:02.0: Direct firmware load for i915/skl_dmc_ver1_26.bin failed with error -2
[   15.109284] i915 0000:00:02.0: Failed to load DMC firmware [https://01.org/linuxgraphics/intel-linux-graphics-firmwares], disabling runtime power management.
[   15.516406] [drm] Initialized i915 1.6.0 20160711 for 0000:00:02.0 on minor 0
[   15.838483] i915 0000:00:02.0: fb0: inteldrmfb frame buffer device
```

I don’t know, why `-x` is passed to the `cp` calls, but copied it for
uniformity.

[1] https://01.org/linuxgraphics/intel-linux-graphics-firmwares

linux-firmware.be0

@@ -71,6 +71,7 @@ mee_install() {
   mkdir -p $D/lib/firmware
   cp -ax ${S}/bnx2  $D/lib/firmware
   cp -ax ${S}/bnx2x $D/lib/firmware
+  cp -ax ${S}/i915  $D/lib/firmware
 }
 
 ## by default this may be 'make install DESTDIR="${D}"'