drm/i915: Fix up address spaces in slow_kernel_write()

Since we now get_user_pages() outside of the mutex prior to performing
the copy, we kmap() the page inside the copy routine and so need to
perform an ordinary memcpy() and not copy_from_user().

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