mesa: fix fallback texture for cube map array

mesa: fix fallback texture for cube map array

mesa was creating a cube map array texture with just one layer, which is
not legal. This caused an assertion failure when using that texture later
in llvmpipe (when enabling cube map arrays) since it verifies the number
of layers in the view is divisible by 6 (the sampling code might well crash
randomly otherwise) with piglit glsl-resource-not-bound CubeArray -fbo -auto.

v2: use appropriately sized texel array...

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)