msm: wfd: Pass proper ion buffer handle to video driver

This change gets ion handle corresponding to output
buffer and passes it to the video driver for ION
kernel mapping.

Change-Id: Ie6783f160029119f0df36f6d4a2e14c737b80edc
CRs-fixed: 508888
Signed-off-by: Deepak Verma <dverma@codeaurora.org>