i965/fs: Rename vp_outputs_written to input_slots_valid.

i965/fs: Rename vp_outputs_written to input_slots_valid.

With the introduction of geometry shaders, fragment inputs will no
longer come exclusively from the vertex shader; sometimes they come
from the geometry shader.  So the name "vp_outputs_written" will
become a misnomer.  This patch renames vp_outputs_written to
input_slots_valid, 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.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>