i965: layout TEXTURE_2D_MULTISAMPLE_ARRAY miptree correctly

Previously this fell through the default path, and so the layer offsets
were bogus. Attempting to render into a nonzero layer would actually
clobber the content of the first layer.

Fixes arb_texture_multisample-texel-fetch-execution-array piglit.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>