swiotlb: remove GFP_DMA hack in swiotlb_alloc_coherent

The callers are supposed to set up the gfp flags appropriately.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>