glsl: assign varying locations to tess shaders when doing SSO

glsl: assign varying locations to tess shaders when doing SSO

GRID Autosport uses SSO shaders. When a tessellation evaluation shader
is passed through this, it triggers assertion failures down the line
with unassigned varying locations. Make sure to do this when the first
shader in the pipeline is not a vertex shader.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>