x86: gart alloc_coherent doesn't need to check NULL device argument

asm/dma-mapping.h guarantees that gart alloc_coherent doesn't get NULL
device argument.

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>