gvt-g support local displays

Feature description: dma-buf is an uniform mechanism to share DMA buffers across different devices and subsystems. dma-buf for intel GVT-g is mainly used to share the vgpu's framebuffer to userspace to leverage userspace graphics stacks to render the framebuffer to the display monitor.
 The main idea is that we create a gem object and set vgpu's framebuffer as its backing storage. Then, export a dma-buf associated with this gem object. With the fd of this dma-buf, userspace can directly handle this buffer.

Applied platform: Skylake, Kabylake

Dependencies: Qemu's support is needed to implement the use cases of local display or remote display via SPICE protocol.

Business value: improve user experience to get local display or remote display of vgpu.

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The dma-buf patches are merged by kernel 4.16 as well as some bug fixing patches, including:
 47419494f216812b42f5d1c5a2984cd46253b4cc drm/i915/gvt: Use KVM r/w to access guest opregion
 412718a10926f77052d8107cb3c5d9dbc96cf8c9 drm/i915/gvt: Keep obj->dma_buf link NULL during exporting
 6ee942d5f7e3e630d3a2517e75969ce5d07c87d6 drm/i915/gvt: Refine dmabuf_obj cleanup process
 82a3b6701ab1e2cdfb64cf163fc6d1a4576329ff drm/i915/gvt: Free dmabuf_obj list in intel_vgpu_dmabuf_cleanup
 4a136d590bd4c579bb3df154363a3e5b51cb36ce drm/i915/gvt: Introduce KBL to dma-buf on Gvt-g
 dfb6ae4e14bd6542ee6caa8f661f269c94e19a9f drm/i915/gvt: Handle orphan dmabuf_objs
 e546e281d33d1fc275651aa06f0659045db67e68 drm/i915/gvt: Dmabuf support for GVT-g
 e20eaa2382e7888a4e06ccb015c476a6fb1fda0c vfio: ABI for mdev display dma-buf operation
 9f31d1063b434c2d54317461d78285b08538c01a drm/i915/gvt: Add framebuffer decoder support
 b851adeac0858c7d257b32eee2142b1519d45ccf drm/i915/gvt: Add opregion support
 a03f395ad78f883df490234366dd4e4fc922d174 drm/i915: Introduce GEM proxy
 274b2462a0493ba73957346fe5fca168e3190b53 drm/i915: Object w/o backing storage is banned by -ENXIO

Currently, Gvt-g repo has three branches which have the dma-buf features:
 1) staging branch
 Has GPU hang issue while booting Linux VM which can be solved by patch:
https://lists.freedesktop.org/archives/intel-gvt-dev/2018-February/003039.html
 2) Q1-release branch (gvt-stable-4.14)
 Still needs more tests.
 3) topic/dmabuf
 This is a branch to show the idea of dma-buf.

[Blog]
 We have a blog to talk about how to use dma-buf feature of Gvg-g and here is the link:
https://01.org/igvt-g/blogs/wangbo85/2018/sharing-guest-framebuffer-host