media: mediabus: Add helpers to convert a ext_pix format to/from a mbus_fmt

Just a new version of v4l2_fill_mbus_format() and v4l2_fill_ext_pix_format()
to deal with the new v4l2_ext_pix_format struct.
This is needed to convert the VIMC driver to the EXT_FMT/EXT_BUF iocts.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
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Changes in v6:
- Rename v4l2_fill_ext_pix_format() to v4l2_fill_ext_pix_format_from_mbus() (Tomasz)

Changes in v4:
- Add helper v4l2_fill_ext_pix_format()
- Rebased on top of media/master (post 5.8-rc1)

Changes in v3:
- Rebased on top of media/master (post 5.4-rc1)

Changes in v2:
- New patch