drm/i915: Use non-atomic kmap for slow copy paths

As we do not have a requirement to be atomic and avoid sleeping whilst
performing the slow copy for shmem based pread and pwrite, we can use
kmap instead, thus simplifying the code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>