drm/i915: Introduce GEM proxy

GEM proxy is a kind of GEM, whose backing physical memory is pinned
and produced by guest VM and is used by host as read only. With GEM
proxy, host is able to access guest physical memory through GEM object
interface. As GEM proxy is such a special kind of GEM, a new flag
I915_GEM_OBJECT_IS_PROXY is introduced to ban host from changing the
backing storage of GEM proxy.

v3:
- update "Reviewed-by". (Joonas)

v2:
- return -ENXIO when pin and map pages of GEM proxy to kernel space.
  (Chris)

Here are the histories of this patch in "Dma-buf support for Gvt-g"
patch-set:

v14:
- return -ENXIO when gem proxy object is banned by ioctl.
  (Chris) (Daniel)

v13:
- add comments to GEM proxy. (Chris)
- don't ban GEM proxy in i915_gem_sw_finish_ioctl. (Chris)
- check GEM proxy bar after finishing i915_gem_object_wait. (Chris)
- remove GEM proxy bar in i915_gem_madvise_ioctl.

v6:
- add gem proxy barrier in the following ioctls. (Chris)
  i915_gem_set_caching_ioctl
  i915_gem_set_domain_ioctl
  i915_gem_sw_finish_ioctl
  i915_gem_set_tiling_ioctl
  i915_gem_madvise_ioctl

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> #v1
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1510555798-21079-2-git-send-email-tina.zhang@intel.com


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114102513.22269-2-chris@chris-wilson.co.uk