MLK-10423: Capture: System hang if capture test app been killed

System will hang if csi unit test mx6s_v4l2_capture.out process
been killed.

In csi capture driver function mx6s_csi_enable/disable is called
by vidioc_streamon/streamoff function.
But when csi unit test process is killed, vidioc_streamon/streamoff
will not been called, and csi function still working even the csi
clock is gated.

Move mx6s_csi_enable/disable function from
mx6s_vidioc_streamon/streamoff function to
mx6s_start_streaming/ mx6s_stop_streaming function to resolve the
issue.

Signed-off-by: Sandor Yu <R01008@freescale.com>