media: exynos4-is: Fix nullptr when no CSIS device present

Not all devices use the CSIS device, some may use the FIMC directly in
which case the CSIS device isn't registered.  This leads to a nullptr
exception when starting the stream as the CSIS device is always
referenced.  Instead, if getting the CSIS device fails, try getting the
FIMC directly to check if we are using the subdev API

Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>