media: dvb: Allow setting buffer to DVR before setting demux source

When DVR buffer is set, the actual source to check the
buffer setting against it should be the respective
DVR source rather than the current source set to demux.

Change-Id: I800eff94e9a05ab6068b1286a3a2d0c024ba1e10
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>