anv: Drop the instance pointer from anv_device

anv: Drop the instance pointer from anv_device

There are very few times when we actually want to fetch the instance
from the anv_device.  We can put up with a bit of pain there in exchange
for strongly discouraging people from doing this in general.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3461>