glsl: Fix interstage uniform interface block link error detection.

Previously, we checked for interstage uniform interface block link
errors in validate_interstage_interface_blocks(), which is only called
on pairs of adjacent shader stages.  Therefore, we failed to detect
uniform interface block mismatches between non-adjacent shader stages.

Before the introduction of geometry shaders, this wasn't a problem,
because the only supported shader stages were vertex and fragment
shaders, therefore they were always adjacent.  However, now that we
allow a program to contain vertex, geometry, and fragment shaders,
that is no longer the case.

Fixes piglit test "skip-stage-uniform-block-array-size-mismatch".