anv: Allow fast-clearing the first slice of a multi-slice image

anv: Allow fast-clearing the first slice of a multi-slice image

Now that we're tracking aux properly per-slice, we can enable this for
applications which actually care.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>