drm/qxl: fix __user annotations

Drop them from u64 fields, tag local variables correctly instead.
While being at it switch the code to use u64_to_user_ptr().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620113916.6967-2-kraxel@redhat.com