i965: Fix gl_VertexID when there are no other vertex inputs.

i965: Fix gl_VertexID when there are no other vertex inputs.

brw_emit_vertices contains special case logic to handle the case where
a vertex shader doesn't read any inputs.  This special case logic was
incorrectly activating in the case were the only vertex input is
gl_VertexID.  As a result, if a shader used gl_VertexID but used no
other inputs, then all vertices got a gl_VertexID of zero.

Fixes oglconform test "ubo-usage advanced.transform_feedback".

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>