drm/i915: always pin hw_id for GVT context

Initially found issue with closed context debug check when pin
hw_id for GVT context, looks we should always pin hw_id for that
as GVT context is fixed for each vGPU life cycle, and we'd also
like to get pinned hw_id e.g for perf reason, etc.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190311023747.1426-1-zhenyuw@linux.intel.com