drm/i915/gtt: Control cache domain of dma_map_page() directly

We already maually control the CPU cache for our page table directories,
so we can tell the dma mapper to skip doing it as well.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706122611.4142-2-chris@chris-wilson.co.uk