drm/i915/gt: Skip trying to unbind in restore_ggtt_mappings

Currently we first to try to unbind the VMA (and lazily rebind on next
use) as an optimisation during restore_ggtt_mappings. Ideally, the only
objects in the GGTT upon resume are the pinned kernel objects which
can't be unbound and need to be restored. As the unbind interferes with
the plan to mark those objects as active for error capture, forgo the
optimisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110110402.1231745-1-chris@chris-wilson.co.uk