drm/exynos: simplify sleep PM ops

PM ops in exynos_drm_drv were split into two separate function as they were
used also by drm device. Since PM ops have been removed from drm device, the
functions can be merged together.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>