drm/i915/gvt: Checking workload's gma earlier

Workload contains RB and WA_CTX which are in ggtt space,
if they aren't in valid ggtt space, the workload shouldn't be
shadowed and scanned. So checking them earlier to avoid shadow
them.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>