drm/i915/gvt: initialize dmabuf mutex in vgpu_create

Currently, the mutex used in GVT dmabuf support is not initialized until
vgpu device is opened. If one vgpu device is opened and then removed, the
mutex will be used in vgpu remove operation without initialization. This
patch initializes the mutex in vgpu create operation to avoid the problem.

Fixes: e546e281("drm/i915/gvt: Dmabuf support for GVT-g")
Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>