drm/i915: Mark shadow batch buffers as purgeable

By adding a new exec_entry flag, we cleanly mark the shadow objects
as purgeable after they are on the active list.

v2:
- Move 'shadow_batch_obj->madv = I915_MADV_WILLNEED' inside _get
  fnc (danvet, from v4 6/7 feedback)

v3:
- Remove duplicate 'madv = I915_MADV_WILLNEED' (danvet, from v6 4/5)

Issue: VIZ-4719
Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-By: Jon Bloomfield <jon.bloomfield@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>