media: imx-mipi-csis: Check csis_fmt validity before use

The find_csis_format() may return NULL in case supported format is not
found, check the return value of find_csis_format() before using the
result to avoid NULL pointer dereference.

Fixes: 11927d0f ("media: imx-mipi-csis: Use V4L2 subdev active state")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>