[media] s5p-fimc: Use pix_mp for the color format lookup

With multi-planar formats fmt.pix_mp member of struct v4l2_format
should be used rather than fmt.pix. Fix find_fmt() function to do
the right thing.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>