[media] dma-mapping: fix dma_common_get_sgtable() conditional compilation

dma_common_get_sgtable() function doesn't depend on
ARCH_HAS_DMA_DECLARE_COHERENT_MEMORY, so it must not be compiled
conditionally.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>