st/mesa: use the correct address generation functions in st_TexSubImage blit

st/mesa: use the correct address generation functions in st_TexSubImage blit

We need to tell the address generation functions about the dimensionality of
the texture to correctly implement the part of Section 3.8.1 (Texture Image
Specification) of the OpenGL 2.1 specification which says:

    "For the purposes of decoding the texture image, TexImage2D is
    equivalent to calling TexImage3D with corresponding arguments
    and depth of 1, except that
      ...
      * UNPACK SKIP IMAGES is ignored."

Fixes a low impact bug that was found by chance while browsing the spec and
extending piglit tests.

Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>