glsl: Return correct number of coordinate components for cubemap array images.

glsl: Return correct number of coordinate components for cubemap array images.

Cubemap array images are unlike cubemap array samplers in that they don't need
an additional coordinate to index individual cubemaps in the array, instead
they behave like a 2D array of 6n layers, with n the number of cubemaps in the
array.  Take this exception into account.

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