i965/fs: Preserve the source register type when doing color writes.
i965/fs: Preserve the source register type when doing color writes.

When rendering to integer color buffers, we need to be careful to use
MRFs of the correct type when emitting color writes.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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