drm/i915/gvt: Add framebuffer decoder support

This patch is to introduce the framebuffer decoder which can decode guest
OS's framebuffer information, including primary, cursor and sprite plane.

v16:
- rebase to 4.14.0-rc6.

v14:
- refine pixel format table. (Zhenyu)

v9:
- move drm format change to a separate patch. (Xiaoguang)

v8:
- fix a bug in decoding primary plane. (Tina)

v7:
- refine framebuffer decoder code. (Zhenyu)

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>