arm64: dma-mapping: don't clear dma ops on bypass detach

On iommu attach and detach we should not be changing the dma ops
if the domain is in bypass.

Fix the iommu detach call to not clear the dma ops if the
domain is in bypass.

CRs-Fixed: 1048740
Change-Id: Ie29e7a6a5375e1ec33b000fa09f01aeb7406c8d9
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>