drm/exynos: g2d: fix system and runtime pm integration

Move code from system sleep pm to runtime pm callbacks to ensure proper
driver state preservation when device is under power domain. Then, use
generic helpers for using runtime pm for system sleep pm.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>