i965: Only use the effective subset of the WM sampler key.

i965: Only use the effective subset of the WM sampler key.

The whole structure is 836 bytes, but if only the first one or two
samplers are used (as is common), the part that matters is only 56 or
108 bytes.  By using just that subset as the key (since the key size
is also part of the key), we improve firefox-talos-gfx performance by
1%.