i965/fs: Fix broken math on values loaded from uniform buffers on gen6.

i965/fs: Fix broken math on values loaded from uniform buffers on gen6.

In a debug build this led to assertion failures, but on a non-debug
build the hardware would just reference the whole vec8 instead of the
same channel 8 times.

Fixes the new piglit glsl-1.40/uniform-buffer/fs-exp2.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57121
Note: This is a candidate for the stable branches
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>