drm/exynos: don't use HW trigger for Exynos5420/5422/5800

Commit a6f75aa1 ("drm/exynos: fimd: add HW trigger support") added
hardware trigger support to the FIMD controller driver. But this broke
the display in at least the Exynos5800 Peach Pi Chromebook.

So until the issue is fixed, avoid using HW trigger for the Exynos5420
based boards and use SW trigger as it was before the mentioned commit.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>