drm/i915/gvt: make gvt oblivious of kvmgt data structures

Instead of defining KVMGT per-device state in struct intel_vgpu
directly, add an indirection. This makes the GVT code oblivious of
what state KVMGT needs to keep.

The intention here is to eventually make it possible to build
hypervisor backends for the mediator, without having to touch the
mediator itself. This is a first step.

v2:
- rebased onto gvt-staging (no conflicts)

Signed-off-by: Julian Stecklina <julian.stecklina@cyberus-technology.de>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200217163858.26496-1-julian.stecklina@cyberus-technology.de