i965/gen6+: Remove VUE map dependency on userclip_active.

i965/gen6+: Remove VUE map dependency on userclip_active.

Previously, on Gen6+, we laid out the vertex (or geometry) shader VUE
map differently depending whether user clipping was active.  If it was
active, we put the clip distances in slots 2 and 3 (where the clipper
expects them); if it was inactive, we assigned them in the order of
the gl_varying_slot enum.

This made for unnecessary recompiles, since turning clipping on/off
for a shader that used gl_ClipDistance might rearrange the varyings.
It also required extra bookkeeping, since it required the user
clipping flag to be provided to brw_compute_vue_map() as a parameter.

With this patch, we always put clip distances at in slots 2 and 3 if
they are written to.  do_vs_prog() and do_gs_prog() are responsible
for ensuring that clip distances are written to when user clipping is
enabled (as do_vs_prog() previously did for gen4-5).

This makes the only input to brw_compute_vue_map() a bitfield of which
varyings the shader writes to, a fact that we'll take advantage of in
forthcoming patches.

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