drm/i915/gvt: Fix build failure after intel_engine_cs change

Change GVT-g code reference for intel_engine_cs from static array to
allocated pointer after commit 3b3f1650 ("drm/i915: Allocate
intel_engine_cs structure only for the enabled engines").

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161018014007.29369-1-zhenyuw@linux.intel.com