drm/i915: Create a full object for mock_ring, v2.

This allows us to finally get rid of all the assumptions that vma->obj
is NULL.

Changes since v1:
- Ensure the mock_ring vma is pinned to prevent a fault.
- Pin it high to avoid failure in evict_for_vma selftest.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211117142024.1043017-3-matthew.auld@intel.com