genirq/msi: Do not use pci_msi_[un]mask_irq as default methods

When we create a generic MSI domain, that MSI_FLAG_USE_DEF_CHIP_OPS
is set, and that any of .mask or .unmask are NULL in the irq_chip
structure, we set them to pci_msi_[un]mask_irq.

This is a bad idea for at least two reasons:
- PCI_MSI might not be selected, kernel fails to build (yes, this is
  legitimate, at least on arm64!)
- This may not be a PCI/MSI domain at all (platform MSI, for example)

Either way, this looks wrong. Move the overriding of mask/unmask to
the PCI counterpart, and panic is any of these two methods is not
set in the core code (they really should be present).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: http://lkml.kernel.org/r/1444760085-27857-1-git-send-email-marc.zyngier@arm.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>