io-mapping: Fixup for different names of writecombine

Somehow architectures can't agree on this. And for good measure make
sure we have a fallback which should work everywhere (fingers
crossed).

v2: Make it compile properly, needs a defined() for the #elif.

Fixes: ac96b556

 ("io-mapping.h: s/PAGE_KERNEL_IO/PAGE_KERNEL/")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: linux-mm@kvack.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160823202233.4681-1-daniel.vetter@ffwll.ch