drm/exynos: Fix suspend/resume support

Commit 7d902c05 ("drm: Nuke drm_atomic_helper_connector_dpms")
removed drm_atomic_helper_connector_dpms() helper saying that it was a dead
code. It was however indirectly used by Exynos DRM driver for implementing
suspend/resume support. To fix this regression (after that patch Exynos DRM
suspend/resume functions became no-ops and hardware fails to suspend),
this patch rewrites them with drm_atomic_helper_suspend/resume() helpers.

Fixes: 7d902c05 ("drm: Nuke drm_atomic_helper_connector_dpms")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>