drm/exynos: correct timing porch conversion

The patch corrects porch calculation. It should
be calculated as a difference between adjacent
respective fields of drm_display_mode.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>