drm/i915/gvt: scan shadow indirect context image when valid

The shadow indirect context image should be only scanned when valid.
So far, Only RCS ring has the shadow indirect context image. This patch
limits the scan logic only for RCS ring.

v2. refine description of the subject
v3. fix alignment. (Zhenyu)

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>