drm/i915: Ensure i915_vma tests do not get -ENOSPC with the locking changes.

Now that we require locking to evict, multiple vmas from the same object
might not be evicted. This is expected and required, because execbuf will
move to short-term pinning by using the lock only. This will cause these
tests to fail, because they create a ton of vma's for the same object.

Unbind manually to prevent spurious -ENOSPC in those mock tests.

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/20211216142749.1966107-8-maarten.lankhorst@linux.intel.com