drm/exynos: set power state variable after enabling clocks and power

Power state variable holds the state of the mixer device.
Power on and power off functions are toggling these variable
at wrong place.

State variable should be changed to true only after Runtime
PM and clocks are enabled. Else it may result to a situation
where mixer registers are accessed with device power enabled.
Similar logic for poweroff sequence.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>