drm/exynos: fimd: harden fimd_calc_clkdiv()

Don't use the vrefresh field of the DRM mode since this
one is supposed to only be used for debug purpose.
Instead use the clock field which should also provide
much more precise information.

Also sanitize the case in which the clock value
should be zero. We then just default to the maximum
clock divisor.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>