i965/vs: When asked to make a dst_reg for a src.xxxx, just write to src.x.

We have several places in our pull constant handling where we make a
temporary src_reg for an int, and then turn it into a dst.  In doing so,
we were writing to the dst.xyzw, so we never register coalesced it with a
later mov from dst.x to real_dst.x.

These extra channels written would be removed if we had channel-wise DCE
in the backend, but we don't.  Fix it for now by just not writing these
extra channels that won't get used.

Reviewed-by: Matt Turner <[email protected]>

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