drm/rockchip: fix for mailbox read validation.

This is basically the same fix as in
commit fa68d4f8 ("drm/rockchip: fix for mailbox read size")
but for cdn_dp_mailbox_validate_receive function.

See patchwork.kernel.org/patch/10671981/ for details.

Signed-off-by: Damian Kos <dkos@cadence.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1542640463-18332-1-git-send-email-dkos@cadence.com