iommu/arm-smmu: Fix enabling of PRIQ interrupt

When an ARM SMMUv3 instance supports PRI, the driver registers
an interrupt handler, but fails to enable the generation of
such interrupt at the SMMU level.

This patches simply moves the enable flags to a variable that
gets updated by the PRI handling code before being written to the
SMMU register.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>