kmssink: Accept underscore in property

Otherwise sdi_mode and similar properties would be canonicalised to
sdi-mode.
6 jobs for kmssink-prop in 36 minutes and 11 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #66832
gst indent

00:00:31

passed #66831
manifest

00:00:24

 
  Build
passed #66833
build fedora x86_64

00:03:06

 
  Test
passed #66834
check fedora

00:16:02

 
  Full Builds
passed #66835
build android arm64 api28

00:02:26

passed #66836
build cerbero fedora x86_64

00:16:28