linker: fix varying linking if SSO program has only gs and fs

linker: fix varying linking if SSO program has only gs and fs

Previously linker did not take in to account case where one would
have only gs and fs (with SSO), patch adds the case by refactoring
code around assign_varying_locations. This makes sure locations for
gs get populated correctly.

This was found with some of the SSO subtests of Martin's upcoming
GetProgramInterfaceiv Piglit test which passes with the patch, no
Piglit regressions.

v2: code cleanups (Martin Peres)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>