media: imx274: actually use IMX274_DEFAULT_MODE

IMX274_DEFAULT_MODE is defined but not used. Start using it, so the
default can be more easily changed without digging into the code.

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>