media: imx274: get rid of mode_index

After restructuring struct imx274_frmfmt, the mode_index field is
still in use only for two dev_dbg() calls in imx274_s_stream(). Let's
remove it and avoid duplicated information.

Replacing the first usage requires some rather annoying but trivial
pointer math. The other one can be removed entirely since it would
print the same value anyway.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>