i965: Shorten sampler loops in key setup.

i965: Shorten sampler loops in key setup.

Now that we have the number of samplers available, we don't need to
iterate over all 16.  This should be particularly helpful for vertex
shaders.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>