drm/exynos: Don't keep dpms state in encoder

This patch removes the dpms state tracking in encoder. This
state is at best confusing and at worst incorrect since the display
drivers can turn on and off without propagating the value.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>