drm/exynos: fimd: Do not use HW trigger for exynos3250

drm/exynos: fimd: Do not use HW trigger for exynos3250

Commit a6f75aa161c5 ("drm/exynos: fimd: add HW trigger support") added
hardware trigger support to the FIMD controller driver. I have tested
but this broke the display in at least the exynos3250 Gear 2. So until
the issue is fixed, avoid using HW trigger for the exynos3250 based
boards and use SW trigger as it was before the mentioned commit.

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>