drm/exynos: fix adjusted_mode pointer in exynos_plane_mode_set

exynos_plane_mode_set should use adjusted_mode from the same atomic state as
plane state. Otherwise it will result in incorrect behavior in case
crtc mode changes.

The patch fixes bug with black console framebuffer in case of command mode
panels.

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