drm/exynos: fixed pm feature for fimd module.

this patch separates fimd specific power on/off function from pm function
and the pm interfaces will call that function for power on or off.
and also removes unnecessary codes of resume function.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>