drm/exynos: Consolidate suspend/resume in drm_drv

This patch removes all of the suspend/resume logic from the individual
drivers and consolidates it in drm_drv. This consolidation reduces the
number of functions which enable/disable the hardware to just one -- the
dpms callback. This ensures that we always power up/down in a consistent
manner.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>