drm/i915: rewrite shmem_pread_slow to use copy_to_user

Like for shmem_pwrite_slow. The only difference is that because we
read data, we can leave the fetched cachelines in the cpu: In the case
that the object isn't in the cpu read domain anymore, the clflush for
the next cpu read domain invalidation will simply drop these
cachelines.

slow_shmem_bit17_copy is now ununsed, so kill it.

With this patch tests/gem_mmap_gtt now actually works.

v2: add __ to copy_to_user_swizzled as suggested by Chris Wilson.

v3: Fixup the swizzling logic, it swizzled the wrong pages.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38115
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>