drm/i915/gvt: Add opregion support

Windows guest driver needs vbt in opregion, to configure the setting
for display. Without opregion support, the display registers won't
be set and this blocks display model to get the correct information
of the guest display plane.

This patch is to provide a virtual opregion for guest. The original
author of this patch is Xiaoguang Chen.

This patch is split from the "Dma-buf support for GVT-g" patch set,
with being rebased to the latest gvt-staging branch.

v3:
- add checking region index during intel_vgpu_rw. (Xiong)

v2:
- refine intel_vgpu_reg_release_opregion. (Xiong)

Here are the previous version comments:

v18:
- unmap vgpu's opregion when destroying vgpu.

v16:
- rebase to 4.14.0-rc6.

Signed-off-by: Bing Niu <bing.niu@intel.com>
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Tested-by: Xiong Zhang <xiong.y.zhang@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>