drm/exynos/mixer: simplify poweron flag

The driver uses bool protected by mutex to track power state.
The patch replaces this combo with single bit and atomic bitops.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>