[media] samples: v4l: from Documentation to samples directory

With the new autoksyms support, we can run into a situation where
the v4l pci skeleton module is the only one using some exported
symbols that get dropped because they are never referenced by
the kernel otherwise, causing a build problem:

ERROR: "vb2_dma_contig_memops" [Documentation/video4linux/v4l2-pci-skeleton.ko] undefined!
ERROR: "vb2_dma_contig_init_ctx_attrs" [Documentation/video4linux/v4l2-pci-skeleton.ko] undefined!
ERROR: "v4l2_match_dv_timings" [Documentation/video4linux/v4l2-pci-skeleton.ko] undefined!
ERROR: "v4l2_find_dv_timings_cap" [Documentation/video4linux/v4l2-pci-skeleton.ko] undefined!
ERROR: "v4l2_valid_dv_timings" [Documentation/video4linux/v4l2-pci-skeleton.ko] undefined!
ERROR: "v4l2_enum_dv_timings_cap" [Documentation/video4linux/v4l2-pci-skeleton.ko] undefined!
ERROR: "vb2_dma_contig_cleanup_ctx" [Documentation/video4linux/v4l2-pci-skeleton.ko] undefined!

Specifically, we do look in the samples directory for users o...