drm/i915: Drop the iomem accessors when writing to the kmapped blt batch

I presumed that we would be writing to the batch through the GTT having
bound it, so I converted it to use iomem. Even later as I spotted that
we didn't even move the batch to the GTT (now an issue since we default
to uncached memory on SNB) I still didn't realise that using iomem for
kmapped memory was incorrect. Fix it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>