glsl: Remove bogus check on return value of link_uniform_blocks().

glsl: Remove bogus check on return value of link_uniform_blocks().

A comment in link_intrastage_shaders(), and an if-test that followed
it, seemed to indicate that link_uniform_blocks() would return a
negative value in the event of an error.  But this is not the
case--all error checking has already been performed by
validate_intrastage_interface_blocks(), and link_uniform_blocks() can
only return unsigned values.

So get rid of the if-test and change the return type of
link_intrastage_shaders() to clarify that it can only return unsigned
values.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>