linker: Assign varying locations geometry shader inputs for SSO

linker: Assign varying locations geometry shader inputs for SSO

Previously only geometry shader outputs would be assigned locations if
the geometry shader was the only stage in the linked program.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: pavol@klacansky.com
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82585
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>