anv: Refactor descriptor copy

anv: Refactor descriptor copy

Refactor descriptor copies to use the existing helper functions instead
of rolling our own. In order to facilitate this, we need to store the
appropriate buffer views for the relevant descriptors internally and
reuse them in the helpers.

Signed-off-by: Rohan Garg <rohan.garg@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14909>