The PRM says "The execution size must be 1."  In 73137997, the
execution size was set to 1 when it should have been BRW_EXECUTE_1
(which maps to 0).  Later, in dc2d3a7f

, JMPI was used for
line AA on gen6 and earlier and we started manually stomping the
exeution size to BRW_EXECUTE_1 in the generator.  This commit fixes the
original bug and makes brw_JMPI just do the right thing.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: 73137997