drm/exynos/mixer: simplify poweron flag
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 <dcc2b772defa18622a340d324a717fd6b8ee2807@samsung.com>
Reviewed-by: Joonyoung Shim <b2207686f8c5343cf0f505ae1d5720ecb0c3202a@samsung.com>
Signed-off-by: Inki Dae <bae4a7a27bbc965064e97edd6e179ac4979db440@samsung.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs