x86: add dma_capable() to replace is_buffer_dma_capable()

dma_capable() eventually replaces is_buffer_dma_capable(), which tells
if a memory area is dma-capable or not. The problem of
is_buffer_dma_capable() is that 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>