drm/omap: add hibernation callbacks

Setting a dev_pm_ops suspend/resume pair but not a set of hibernation
functions means those pm functions will not be called upon hibernation.

Fix this by using SIMPLE_DEV_PM_OPS, which appropriately assigns the
suspend and hibernation handlers and move
omap_drm_suspend/omap_drm_resume under CONFIG_PM_SLEEP to avoid build
warnings.

Signed-off-by: Grygorii Strashko <Grygorii.Strashko@linaro.org>
[tomi.valkeinen@ti.com: fix conflict, clean up description]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>