drm/exynos: mixer: simplify vp_video_buffer()

DRM core already checks in drm_atomic_plane_check() if the
pixelformat is valid. Hence we can drop the default case of
the switch statement and collapse most of the code.

Also rename the two booleans to reflect what true/false
actually means, and to avoid mixing CrCb/NV21 descriptions.

No functional change.

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