remove is_buffer_dma_capable()

is_buffer_dma_capable() was replaced with dma_capable().

is_buffer_dma_capable() tells if a buffer is dma-capable or
not. However, it doesn't take a pointer to struct device so it doesn't
work for POWERPC.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Becky Bruce <beckyb@kernel.crashing.org>