irqchip: gicv3-its: Fix ITS CPU init

We skip initialisation of ITS in case the device-tree has no
corresponding description, but we are still accessing to ITS bits while
setting CPU interface what leads to the kernel panic:

ITS: No ITS available, not enabling LPIs
CPU0: found redistributor 0 region 0:0x000000002f100000
Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = ffffffc0007fb000
[00000000] *pgd=00000000fc407003, *pud=00000000fc407003, *pmd=00000000fc408003, *pte=006000002f000707
Internal error: Oops: 96000005 [parallella#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.19.0-rc2+ analogdevicesinc#318
Hardware name: FVP Base (DT)
task: ffffffc00077edb0 ti: ffffffc00076c000 task.ti: ffffffc00076c000
PC is at its_cpu_init+0x2c/0x320
LR is at gic_cpu_init+0x168/0x1bc

It happens in gic_rdists_supports_plpis() because gic_rdists is NULL.
The gic_rdists is set to non-NULL only when ITS node is presented in
the device-tree.

Fix this by moving the call to gic_rdists_supports_plpis() inside the
!list_empty(&its_nodes) block, because it is that list that guards the
validity of the rest of the information in this driver.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1425659870-11832-2-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>

drivers/irqchip/irq-gic-v3-its.c

@@ -1382,12 +1382,11 @@ static bool gic_rdists_supports_plpis(void)
 
 int its_cpu_init(void)
 {
-	if (!gic_rdists_supports_plpis()) {
-		pr_info("CPU%d: LPIs not supported\n", smp_processor_id());
-		return -ENXIO;
-	}
-
 	if (!list_empty(&its_nodes)) {
+		if (!gic_rdists_supports_plpis()) {
+			pr_info("CPU%d: LPIs not supported\n", smp_processor_id());
+			return -ENXIO;
+		}
 		its_cpu_init_lpis();
 		its_cpu_init_collection();
 	}