drm/exynos: fixed converting between display mode and timing

missing members are added into converting function between timing and display
mode and refresh rate of display mode is calculated by drm mode function.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>