drm/i915/gvt: Dmabuf support for GVT-g

This patch introduces a guest's framebuffer sharing mechanism based on
dma-buf subsystem. With this sharing mechanism, guest's framebuffer can
be shared between guest VM and host.

v17:
- modify VFIO_DEVICE_GET_GFX_DMABUF interface. (Alex)

v16:
- add x_hot and y_hot. (Gerd)
- add flag validation for VFIO_DEVICE_GET_GFX_DMABUF. (Alex)
- rebase 4.14.0-rc6.

v15:
- add VFIO_DEVICE_GET_GFX_DMABUF ABI. (Gerd)
- add intel_vgpu_dmabuf_cleanup() to clean up the vGPU's dmabuf. (Gerd)

v14:
- add PROBE, DMABUF and REGION flags. (Alex)

v12:
- refine the lifecycle of dmabuf.

v9:
- remove dma-buf management. (Alex)
- track the dma-buf create and release in kernel mode. (Gerd) (Daniel)

v8:
- refine the dma-buf ioctl definition.(Alex)
- add a lock to protect the dmabuf list. (Alex)

v7:
- release dma-buf related allocations in dma-buf's associated release
  function. (Alex)
- refine ioctl interface for querying plane info or create dma-buf.
  (Alex)

v6:
- align the dma-buf life cycle with the vfio device. (Alex)
- add the dma-buf related operations in a separate patch. (Gerd)
- i915 related changes. (Chris)

v5:
- fix bug while checking whether the gem obj is gvt's dma-buf when user
  change caching mode or domains. Add a helper function to do it.
  (Xiaoguang)
- add definition for the query plane and create dma-buf. (Xiaoguang)

v4:
- fix bug while checking whether the gem obj is gvt's dma-buf when set
  caching mode or doamins. (Xiaoguang)

v3:
- declare a new flag I915_GEM_OBJECT_IS_GVT_DMABUF in drm_i915_gem_object
  to represent the gem obj for gvt's dma-buf. The tiling mode, caching
  mode and domains can not be changed for this kind of gem object. (Alex)
- change dma-buf related information to be more generic. So other vendor
  can use the same interface. (Alex)

v2:
- create a management fd for dma-buf operations. (Alex)
- alloc gem object's backing storage in gem obj's get_pages() callback.
  (Chris)

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>