iommu/ipmmu-vmsa: Only call platform_get_irq() when interrupt is mandatory

As platform_get_irq() now prints an error when the interrupt does not
exist, calling it gratuitously causes scary messages like:

    ipmmu-vmsa e6740000.mmu: IRQ index 0 not found

Fix this by moving the call to platform_get_irq() down, where the
existence of the interrupt is mandatory.

Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>