decklinkvideosrc: Don't report that we have signal until we know for sure

Previously we would've reported that there is signal unless we know for
sure that we don't have signal. For example signal would've been
reported before the device is even opened.

Now keep track whether the signal state is unknown or not and report no
signal if we don't know yet. As before, only send an INFO message about
signal recovery if we actually had a signal loss before.
16 jobs for master in 87 minutes and 48 seconds (queued for 1 minute and 46 seconds)
Status Job ID Name Coverage
  Preparation
passed #421043
gst indent

00:00:34

passed #421042
manifest

00:00:26

 
  Build
passed #421049
build cerbero android universal

00:15:57

passed #421050
build cerbero cross win32

00:14:49

passed #421051
build cerbero cross win64

00:06:22

passed #421048
build cerbero fedora x86_64

00:07:42

manual #421053
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #421052
gst-macos-10.14
build cerbero macos x86_64

00:06:39

passed #421044
build fedora x86_64

00:07:38

passed #421045
build nodebug fedora x86_64

00:08:02

passed #421046
build static fedora x86_64

00:12:46

passed #421047
build static nodebug fedora x86_64

00:14:56

 
  Test
passed #421054
check fedora

00:04:08

passed #421055
integration testsuites fedora

00:10:32

 
  Integrate
passed #421056
android universal examples

00:07:37

passed #421057
documentation

00:04:57