media: dvb: Fix demux event notification for section filters

Section event notifications should be sent to all of the filters
associated with a section feed.
This fixes a few cases in which a notification was sent only to
one filter, causing the other filters to miss section events.

Change-Id: I4cc6271ece1cf89f81f7c17680e7852be2f56f2d
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>