drm/i915/gvt: Audit the command buffer address

The command buffer address in context like ring buffer base address
and wa_ctx address need to be audit to make sure they are in the
valid GGTT range.

Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>