drm/exynos/decon5433: fix software trigger mask

The patch fixes copy/paste bug introduced during code refactoring.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: b93c2e8b ("drm/exynos/decon5433: configure sysreg in case of hardware trigger")Fixes:
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>