media: imx6-mipi-csi2: use pre_streamon callback to set sensor into LP11

Step 5 expects that the sensor is in LP11 mode. Use the new
pre_streamon callback to signal the sensor that it should switch into
LP11.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>