arm64/dma-mapping: Remove extraneous null-pointer checks

The current null-pointer check in __dma_alloc_coherent and
__dma_free_coherent is not needed anymore since the
__dma_alloc/__dma_free functions won't be called if !dev (dummy ops will
be called instead).

Change-Id: I037c6fab4626dd0fc3ab13313a202ad14c8454b0
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>