iommu/arm-smmu: Disable interrupts whilst holding the cmdq lock

The cmdq lock is taken whenever we issue commands into the command queue,
which can occur in IRQ context (as a result of unmap) or in process
context (as a result of a threaded IRQ handler or device probe).

This can lead to a theoretical deadlock if the interrupt handler
performing the unmap hits whilst the lock is taken, so explicitly use
the {irqsave,irqrestore} spin_lock accessors for the cmdq lock.

Tested-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>