i965: Use SET_FIELD to safety check our x/y offsets in blits.

i965: Use SET_FIELD to safety check our x/y offsets in blits.

The earlier assert made sure that our math didn't exceed our bounds, but
this makes sure that we don't overflow from the high bits X into the low
bits of Y.  We've already put checks in intel_miptree_blit(), but I've
wanted to expand the type in our protoype from short to uint32_t, and we
could get in trouble with intel_emit_linear_blit() if we did.

v2: Add Ken's comment about the funny language extension used.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com> (v1)