powerpc/pmu/fsl: fix is_nmi test for irq mask change

When soft enabled was changed to irq disabled mask, this test missed
being converted (although the equivalent book3s test was converted).

The PMU drivers consider it an NMI when they take a PMI while general
interrupts are disabled. This change restores that behaviour.

Fixes: 01417c6c ("powerpc/64: Change soft_enabled from flag to bitmask")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>