drm/exynos: mixer: convert booleans to flags in mixer context

The mixer context struct already has a 'flags' field, so
we can use it to store the 'interlace', 'vp_enabled' and
'has_sclk' booleans.
We use the non-atomic helper functions to access these bits.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>