msm: wfd: Add an dummy encoder subdevice for debugging

Created a dummy encoder subdevice that just copies the input buffer into
the output buffer.  This is useful when we'd like to examine the MDP
output, which is normally very difficult as the buffers associated with
the MDP output aren't exposed to userspace.  Since the dummy enc-subdev,
copies the input buffer into the output buffer, which is exposed to
userspace, we can easily dump the MDP data.

Change-Id: If855b5cd78ae76a781e5709d2bc80167bfadfbf8
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>