v4l2src: preserve dmabuf caps features

The dmabuf caps features have been lost while sorting candidate caps
to get nearest preference caps. It separates caps into GstStructure,
merged them back into cap but the caps features was missing in these
steps. This change makes to preserve the dmabuf caps features after
sorting the GstStructures of the caps.
13 jobs for v4l2 in 64 minutes and 2 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Preparation
passed #237135
gst indent

00:00:35

passed #237134
manifest

00:00:36

 
  Build
passed #237136
build fedora x86_64

00:07:01

passed #237137
build nodebug fedora x86_64

00:06:47

 
  Test
passed #237138
check fedora

00:04:53

passed #237139
integration testsuites fedora

00:27:12

 
  Full Builds
passed #237141
build cerbero android universal

00:16:07

passed #237142
build cerbero cross win32

00:12:02

passed #237143
build cerbero cross win64

00:04:11

passed #237140
build cerbero fedora x86_64

00:10:45

 
  Full Tests
passed #237145
valgrind bad

00:06:10

passed #237144
valgrind good

00:07:40

 
  Apps
passed #237146
android universal examples

00:05:23