drm/exynos: mixer: remove buffer count handling in vp_video_buffer()

The video processor (VP) supports four formats: NV12, NV21 and its
tiled variants. All these formats are bi-planar, so the buffer
count in vp_video_buffer() is always 2.

Also properly exit if we're called with an invalid (non-VP) pixelformat.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>