iommu: Initialize domain->handler in iommu_domain_alloc()

Since it is not guaranteed that an iommu driver initializes in its
domain_init() function, it must be initialized with NULL to prevent
calling a function in an arbitrary location when iommu fault occurred.

Change-Id: I1001bc6382cfe1e6cf8f311a822380f4151f8391
Signed-off-by: KyongHo Cho <pullip.cho@samsung.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
[stepanm@codeaurora.org: rebased to before the file moved]
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>