drm/exynos/decon5433: always do sw-trigger when vblanks enabled

When vblanks are enabled userspace and/or kernel can expect vblank
interrupt at declared period of time. To generate vblank interrupt
image transfer must be triggered. This patch fixes vblank timeouts
in case of sw-trigger mode.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>