media: rockchip: rga: use pixelformat to find format

Use the pixelformat instead of the v4l2_format to find the rga_fmt. This
avoids knowing the structure and type of v4l2_format in rga_fmt_find and
simplifies the function.

Also cleanup the users of the function. In try_fmt always return the
found pixel format to make sure that the pixel format is always set.
Thus, we can be sure that we will find the rga_fmt in s_fmt and can drop
the check if a given format has been found.

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