drm/exynos/decon5433: use mode info stored in CRTC to detect i80 mode

Since panel's mode of work is propagated properly from panel to DECON,
there is no need to use redundant private device tree property.
The only issue with such approach is that check for required interrupts
should be postponed until panel communicate its requirements, ie to
mode validation phase - mode_valid callback.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>