Check if ppgtt is valid for context when init reg state. For gvt
context which has no i915 allocated ppgtt, failed to check that
would cause kernel null ptr reference error.

v2: remove !48bit ppgtt case as we'll always update before submit (Chris)
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109131453.3943-1-zhenyuw@linux.intel.com