glsl/linker: Ensure the first stage of an SSO pipeline has input locs assigned

glsl/linker: Ensure the first stage of an SSO pipeline has input locs assigned

Previously an SSO pipeline containing only a tessellation control shader
and a tessellation evaluation shader would not get locations assigned
for the TCS inputs.  This would lead to assertion failures in some
piglit tests, such as arb_program_interface_query-resource-query.

That piglit test still fails on some tessellation related subtests.
Specifically, these subtests fail:

'GL_PROGRAM_INPUT(tcs) active resources' expected 2 but got 3
'GL_PROGRAM_INPUT(tcs) max length name' expected 12 but got 16
'GL_PROGRAM_INPUT(tcs,tes) active resources' expected 2 but got 3
'GL_PROGRAM_INPUT(tcs,tes) max length name' expected 12 but got 16
'GL_PROGRAM_OUTPUT(tcs) active resources' expected 15 but got 3
'GL_PROGRAM_OUTPUT(tcs) max length name' expected 23 but got 12

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Cc: mesa-stable@lists.freedesktop.org