drm/i915/gvt: Adding ppgtt to GVT GEM context after shadow pdps settled.

Windows guest can't run after force-TDR with host log:
...
gvt: vgpu 1: workload shadow ppgtt isn't ready
gvt: vgpu 1: fail to dispatch workload, skip
...

The error is raised by set_context_ppgtt_from_shadow(), when it checks
and found the shadow_mm isn't marked as shadowed.

In work thread before each submission, a shadow_mm is set to shadowed in:
shadow_ppgtt_mm()
<-intel_vgpu_pin_mm()
<-prepare_workload()
<-dispatch_workload()
<-workload_thread()
However checking whether or not shadow_mm is shadowed is prior to it:
set_context_ppgtt_from_shadow()
<-dispatch_workload()
<-workload_thread()

In normal case, create workload will check the existence of shadow_mm,
if not it will create a new one and marked as shadowed. If already exist
it will reuse the old one. Since shadow_mm is reused, checking of shadowed
in set_context_ppgtt_from_shadow() actually always see the state set in
creation, but not the state set in intel_vgpu_pin_mm().

When force-TDR, all engines are reset, since it's not dmlr level, all
ppgtt_mm are invalidated but not destroyed. Invalidation will mark all
reused shadow_mm as not shadowed but still keeps in ppgtt_mm_list_head.
If workload submission phase those shadow_mm are reused with shadowed
not set, then set_context_ppgtt_from_shadow() will report error.

Pin for context after shadow_mm pinned and shadow pdps settled.

v2:
Move set_context_ppgtt_from_shadow() after prepare_workload(). (zhenyu)
v3:
Move set_context_ppgtt_from_shadow() after shadow pdps updated.(zhenyu)

Fixes: 4f15665c

 ("drm/i915: Add ppgtt to GVT GEM context")
Cc: stable@vger.kernel.org
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>