i965/miptree: Set logical_depth0 == 6 for cube maps

i965/miptree: Set logical_depth0 == 6 for cube maps

This matches what we do for cube maps where logical_depth0 is in number of
face-layers rather than number of cubes.  This does mean that we will
temporarily be setting the surface bounds too loose for cube map textures
but we are already setting them too loose for cube arrays and we will be
fixing that in the next commit anyway.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Cc: "12.0 11.2 11.1" <mesa-stable@lists.freedesktop.org>