drm: tegra: fix front_porch <-> back_porch mixup

Fixes wrong picture offset observed when using HDMI output with a
Technisat HD TV.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Mark Zhang <markz@nvidia.com>
Tested-by: Mark Zhang <markz@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>