media: rockchip: rga: use v4l2_m2m_buf_copy_metadata

The v4l2_m2m_buf_copy_metadata function correctly copies the metadata of
the buffer. Use that function instead of open-coding the metadata copy.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>