drm/exynos: Remove exynos_plane_dpms() call with no effect
exynos_plane_dpms(DRM_MODE_DPMS_ON) calls the win_enable()'s callback
from the underlying layer. However neither one of these layers implement
win_enable() - FIMD, Mixer and VIDI. Thus the call to exynos_plane_dpms()
is pointless.