anv: Replace aux_surface.isl.size_B checks with aux_usage checks

anv: Replace aux_surface.isl.size_B checks with aux_usage checks

Now that aux_usage has a unified meaning, aux_usage == NONE if and only
if aux_surface.isl.size_B > 0.  In most of these cases, the question
we're asking is "does have compression?" and not "have we allocated an
aux surface for compression?".

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3556>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3556>