powerpc/pseries/msi: Avoid reading PCI device registers in reduced power states

When a system is being suspended to RAM, the PCI devices are also
suspended and the PPC code ends up calling pseries_msi_compose_msg() and
this triggers the BUG_ON() in __pci_read_msi_msg() because the device at
this point is in reduced power state. In reduced power state, the memory
mapped registers of the PCI device are not accessible.

To replicate the bug:
1. Make sure deep sleep is selected
	# cat /sys/power/mem_sleep
	s2idle [deep]

2. Make sure console is not suspended (so that dmesg logs are visible)
	echo N > /sys/module/printk/parameters/console_suspend

3. Suspend the system
	echo mem > /sys/power/state

To fix this behaviour, read the cached msi message of the device when the
device is not in PCI_D0 power state instead of touching the hardware.

Fixes: a5f3d2c1 ("powerpc/pseries/pci: Add MSI domains")
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250305090237.294633-1-gautam@linux.ibm.com