media: staging/intel-ipu3: css: simplify expression

An array index computed inside square brackets complicates the code
and also extends the line beyond 80 character. Add new variable to
compute array index separately and use it as an index during assignment.

Signed-off-by: Deepak R Varma <mh12gx2825@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>