irqchip/gic-v3-its: Fix double ICC_EOIR write for LPI in EOImode==1

When the GIC is using EOImode==1, the EOI is done immediately,
leaving the deactivation to be performed when the EOI was
previously done.

Unfortunately, the ITS is not aware of the EOImode at all, and
blindly EOIs the interrupt again. On most systems, this is ignored
(despite being a programming error), but some others do raise a
SError exception as there is no priority drop to perform for this
interrupt.

The fix is to stop trying to be clever, and always call into the
underlying GIC to perform the right access, irrespective of the
more we're in.

[Marc: Reworked commit message]

Fixes: 0b996fd3 ("irqchip/GICv3: Convert to EOImode == 1")
Cc: stable@vger.kernel.org
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Ashok Kumar <ashoks@broadcom.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>