media: staging: ipu3-imgu: Initialise height_per_slice in the stripes

While playing with low resolutions for the grid, it appeared that
height_per_slice is not initialised if we are not using both stripes for
the calculations. This pattern occurs three times:
- for the awb_fr processing block
- for the af processing block
- for the awb processing block

The idea of this small portion of code is to reduce complexity in
loading the statistics, it could be done also when only one stripe is
used. Fix it by getting this initialisation code outside of the else()
test case.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>