drm/exynos: update crtc to plane safely

if old_crtc isn't same as encoder->crtc then it means that
user changed crtc id to another one so a plane to old_crtc
should be disabled so that current plane can be updated safely
and plane->crtc should be set to new crtc(encoder->crtc)

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>