drm/exynos: remove mode property of exynos crtc

This was added by commit 3b8d1cf8 ("drm/exynos: add property for
crtc mode"). Currently we can control a plane used for crtc using
primary plane by universal plane feature. Stop to use non-standard
property to control primary plane.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>