drm/i915/gvt: Pin the per-engine GVT shadow contexts

Our eventual goal is to rid request construction of struct_mutex, with
the short term step of lifting the struct_mutex requirements into the
higher levels (i.e. the caller must ensure that the context is already
pinned into the GTT). In this patch, we pin GVT's shadow context upon
allocation and so keep them pinned into the GGTT for as long as the
virtual machine is alive, and so we can use the simpler request
construction path safe in the knowledge that the hard work is already
done.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190426163336.15906-1-chris@chris-wilson.co.uk