drm/vc4: Start using u64_to_user_ptr.

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>
(cherry picked from commit 95d7cbcb6fbe3885d7a968cc86b98a08466742af)

[inki.dae: apply from rpi3-4.9.y]
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Change-Id: Ie95aa6eebe152b453c3d6f5dd330c79af3b12353