drm/exynos: fimd: replace struct fb_videomode with videomode

The patch replaces all occurrences of struct fb_videomode by
more accurate struct videomode. The change allows to remove
mode conversion function and simplifies clock divider calculation.
Clock configuration is moved to separate function.

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