mesa: Define helper function to get the number of texture layers.

mesa: Define helper function to get the number of texture layers.

And to check if it can have layers at all.  This will be used by the
implementation of ARB_shader_image_load_store.

v2: Fix constness of texobj argument, use assert and return reasonable
    default rather than calling unreachable() in default switch case.

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>