i965 Gen6+: De-compact clip plane constants for old VS backend.

i965 Gen6+: De-compact clip plane constants for old VS backend.

In commit 018ea68d8780ab5baeef0b8122b8410e5e55ae6d, when I
de-compacted clip planes on Gen6+, I updated both the old and new VS
back-ends to reflect the change in how clip planes are stored, but I
failed to change the code in gen6_vs_state.c that uploads clip plane
constants when using the old VS back-end.

As a result, if the set of enabled clip planes wasn't contiguous
starting with 0, then clipping would not occur properly.  This patch
corrects gen6_vs_state.c to upload clip plane constants in the new
de-compacted form.

This only affects the old VS back-end (which is used for
fixed-function and ARB vertex programs, not for GLSL vertex shaders).

Fixes Piglit test fixed-clip-enables.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=41603

Reviewed-by: Eric Anholt <eric@anholt.net>