i965/fs: Rename vp_outputs_written to input_slots_valid.

With the introduction of geometry shaders, fragment inputs no longer
come exclusively from the vertex shader; sometimes they come from the
geometry shader.  So the name "vp_outputs_written" becomes a misnomer.
Renamed to reflect the true meaning of the bitfield from the fragment
shader's point of view: it indicates which of the possible input slots
contain valid data that was written by the previous shader stage.
WIP: add GS visitors