iommu: iommu-debug: fix null-ptr-deref in atos write

iommu_debug_atos_write checks for invalid phys address before atos write.
If done without domain attach this can result in null-ptr-deref.

example call trace:

iommu_iova_to_phys+0x10
iommu_debug_atos_write+0x74
__vfs_write+0x60
vfs_write+0xe4
ksys_write+0x78
__arm64_sys_write+0x1c
el0_svc_common+0xbc
el0_svc_handler+0x68
el0_svc+0x8

Fix this by checking for valid domain attached.

Change-Id: Ib35ae624ed5a9d18c4772697df73887dba8b4bb6
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>