libmix: allocate the extra buffer for WiDi dynamically

BZ: 175667

When WiDi is connected, the property "media.widi.enabled" is set and
6 extra buffers are allocated.

Change-Id: Ib9ceb0be85cbc0e806b7413b9f7a6e2a18b3031f
Signed-off-by: Dan Liang <dan.liang@intel.com>
4 files changed