i965/fs: Use LD messages for pre-gen7 varying-index uniform loads

i965/fs: Use LD messages for pre-gen7 varying-index uniform loads

This comes at a minor performance cost at the moment (-3.2% +/- 0.2%, n=14 on
my GM45 forced to load all uniforms through the varying-index path), but we
get a whole vec4 at a time to reuse in the next commit.

v2: Fix comment about channels in the other message.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
NOTE: This is a candidate for the 9.1 branch.