drm/i915: GVT-g driver depends on 64BIT kernel

We currently don't support GVT-g driver on i386 kernel.
Add explicit dependence on 64bit kernel.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>