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.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Inki Dae <inki.dae@samsung.com>