drm/i915/gvt: Wean gvt off using dev_priv

Teach gvt to use intel_gt directly as it currently assumes direct HW
access.

[Zhenyu: rebase, fix compiling]

Cc: Ding Zhuocheng <zhuocheng.ding@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200304032307.2983-3-zhenyuw@linux.intel.com