drm/i915: Create a dummy object for gen6 ppgtt

We currently have to special case vma->obj being NULL because
of gen6 ppgtt and mock_engine. Fix gen6 ppgtt, so we may soon
be able to remove a few checks. As the object only exists as
a fake object pointing to ggtt, we have no backing storage,
so no real object is created. It just has to look real enough.

Also kill pin_mutex, it's not compatible with ww locking,
and we can use the vm lock instead.

v2:
  - Drop IS_SHRINKABLE and shorten overly long line
v3:
  - Checkpatch fix for alignment

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/20211117142024.1043017-2-matthew.auld@intel.com