iommu: dma-mapping-fast: Remove dma_supported op

Assume a configuration with a device behind an iommu.
The device is able to access all addresses that the iommu is able to
access. In current configurations, the iommu and cpu can access the same
address space, so dma_supported should always return true. Since this is
the default behavior, remove the function.

Change-Id: Ia6bf4029c025f21502c9345ffd781f17f8706851
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>