drm/exynos: initialize the buf_num in vp_video_buffer

The buf_num in vp_video_buffer() should be 1 or 2, but it is not
initialized, and only set to 2 in NV12M or NV12MT cases.
So this patch initializes the buf_num with 1 as default.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>