gpu: ipu-v3: image-convert: calculate tile dimensions and offsets outside fill_image

This will allow to calculate seam positions after initializing the
ipu_image base structure but before calculating tile dimensions.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Steve Longerbeam <slongerbeam@gmail.com>
Tested-by: Steve Longerbeam <slongerbeam@gmail.com>