st/mesa: only minify depth for 3d targets

st/mesa: only minify depth for 3d targets

We make sure that that image depth matches the level's depth before
copying it into place. However we should only be minifying the first
level's depth for 3d textures - array textures have the same depth for
all levels.

This fixes tests such as
dEQP-GLES3.functional.texture.specification.texsubimage3d_depth.* and I
suspect account for a number of other odd situations I've run into where
level > 0 of array textures was messed up.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>