drm/i915/gvt: fix an error string format

It is better to use %p format for void pointers instead of casting them
because a void* is not necessary a 64 bits value.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>