drm/exynos/fimc: simplify pre-scaler ratio calculation

The patch replaces dedicated function for scaling ratio
calculation by fls calls.

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

diff --cc drivers/gpu/drm/exynos/exynos_drm_fimc.c
Simple merge