drm/vc4: Start using u64_to_user_ptr.

Chris Wilson pointed out this little cleanup in a review of new code,
so let's fix up the code I was copying from.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170725182718.31468-1-eric@anholt.net


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>