drm/exynos: fix display on issue

When crtc_funcs->dpms callback is called, exynos_crtc->dpms
and exynos_encoder->dpms are changed to new mode. But if user
requests dpms mode operation, OFF -> ON, when crtc's dpms callback
is called, exynos_encoder->dpms is also changed to ON. This
makes encoder's dpms callback call be ignored so display power
couldn't become on again.

This patch removes exynos_encoder->dpms changing and adds 'updated'
variable to exynos_drm_encoder structure to avoid duplicated overlay
updating.

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