drm/exynos/fimd: add clock rate checking

In case of some platforms fimd clocks can be configured to
very low values, as a result refresh rate can be very low and
driver/drm-core will timeout waiting for vblanks, it will result
in premature removal of framebuffers and will cause oopses.
The patch adds atomic_check callback to fimd to prevent setting
such modes.

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