iommu/amd: Factor out setting the remap table for a devid

Setting the IRQ remap table for a specific devid (or its alias devid)
includes three steps. Those three steps are always repeated each time
this is done.
Introduce a new helper function, move those steps there and use that
function instead. The compiler can still decide if it is worth to
inline.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>