drm/i915/gvt: Fix out-of-bounds buffer write into opregion->signature[]

sparse spots

drivers/gpu/drm/i915/gvt/opregion.c:234 alloc_and_init_virt_opregion() error: memcpy() 'header->signature' too small (16 vs 17)

as gvt is indeed trying to memcpy a string longer than the signature[].

Fixes: b2d6ef70

 ("drm/i915/gvt: Let each vgpu has separate opregion memory")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Xiong Zhang <xiong.y.zhang@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: intel-gvt-dev@lists.freedesktop.org
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>