drm/i915: Replace some open-coded i915_coherent_map_type()

A few callsites were deciding on using WC or WB maps based on
HAS_LLC(), so replace them with the equivalent helper function
i915_coherent_map_type().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001194447.29910-1-chris@chris-wilson.co.uk