msm: wfd: For secure WFD add support to use video39

The commit changes the interface by which userspace clients can use
secure WFD.  Previously, /dev/video38 was opened and client set
V4L2_CID_MPEG_VIDC_VIDEO_SECURE.  Now, the wfd driver also makes use of
the already existing /dev/video39 which is always in secure mode.  This
interface aligns the driver more closely with the interface on A-family
chipsets.

Change-Id: Iab6f4f77b4ee73eada9ec7542e4e456e6d502248
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>