mesa: optimise interleaved sso validation

mesa: optimise interleaved sso validation

Now that we have a linked_stages bitfield we can use this
to check if the program is used at a later stage.

This change is also required to be able to use gl_program
rather than gl_shader_program in the CurrentProgram array.

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