drm/i915/gvt: Fix one gvt_vgpu_error() use in dmabuf.c

Fix below warning with proper usage.

CHECK   drivers/gpu/drm/i915//gvt/dmabuf.c
drivers/gpu/drm/i915//gvt/dmabuf.c:462 intel_vgpu_get_dmabuf() error: 'vgpu' dereferencing possible ERR_PTR()

Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>