drm/i915: Fix lock order reversal in shmem pwrite path.

Like the GTT pwrite path fix, this uses an optimistic path and a
fallback to get_user_pages.  Note that this means we have to stop using
vfs_write and roll it ourselves.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>