drm/i915: Ensure gem_contexts selftests work with unbind changes, v2.

In the next commits, we may not evict when refcount = 0.

igt_vm_isolation() continuously tries to pin/unpin at same address,
but also calls put() on the object, which means the object may not
be unpinned in time.

Instead of this, re-use the same object over and over, so they can
be unbound as required.

Changes since v1:
- Fix cleaning up obj_b on failure. (Matt)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211216142749.1966107-7-maarten.lankhorst@linux.intel.com