drm/exynos/hdmi: move PLL stabilization check code to separate function

The patch moves PLL stabilization check to separate function, adjust timeout
parameters and de-duplicates code common for both HW variants.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>