PCI: designware: Factor out MSI msg setup

Factor out the PCI MSI message setup from the single MSI setup function.
This will be reused by the multivector MSI setup.

No functional change yet.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>