media: dvb: Add support for external demux buffer allocation

demux input/output buffers were allocated internally by the driver,
for MPQ use-cases there's a need for user-space MPQ player to
allocate the buffers through ION driver and set them to demux
similar to the way other kinds of buffers are managed
in android multimedia stack for video

Change-Id: Ic28a279c0841fee12008cea7beac58f82409b9f9
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>
Signed-off-by: Neha Pandey <nehap@codeaurora.org>