[media] s5p-fimc: Fix bytesperline value for V4L2_PIX_FMT_YUV420M format

Make sure bytesperline for Cb, Cr planes for V4L2_PIX_FMT_YUV420M
format is half of the Y plane value, rather than having same
bytesperline for all planes.
While at it, simplify the bytesperline parameter handling by
storing it when image format is set and returning those values
when getting the format, instead of recalculating bytesperline
from intermediate parameters.

Reported-by: Sebastian Dröge <sebastian.droege@collabora.co.uk>
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>