drm/exynos: fix overlay updating issue

Chagelog v2:
Move encoder's dpms updating into exynos_drm_encoder_commit
function because when crtc's dpms is updated, encoder's dpms
is updated also. This would induce the issue that encoder
isn't disabled after crtc is disabled.

Changelog v1:
This patch fixes a issue that overlay data aren't applied
to real hardware when dpms off goes to on after setcrtc
was requested like below,
    dpms off -> setcrtc -> dpms off -> dpms on

For this, it makes encoder's dpms to be updated when
setcrtc is requested.

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