v4l2object: Set minimum buffers to 2

All the element requires at least two buffers. This is not used for RW mode.

sys/v4l2/gstv4l2object.h

@@ -38,7 +38,7 @@ typedef struct _GstV4l2Xv GstV4l2Xv;
 #include <gstv4l2bufferpool.h>
 
 /* size of v4l2 buffer pool in streaming case */
-#define GST_V4L2_MIN_BUFFERS 1
+#define GST_V4L2_MIN_BUFFERS 2
 
 /* max frame width/height */
 #define GST_V4L2_MAX_SIZE (1<<15) /* 2^15 == 32768 */