gpu: ipu-v3: image-convert: fix output seam valid interval

This fixes a failure to determine any seam if the output size is
exactly 1024 multiplied by the number of tiles in a given direction.
In that case an empty interval out_start == out_end is being passed
to find_best_seam, which looks for a seam out_start <= x < out_end.

Also reduce the interval for all but the left column / top row, to
avoid returning position 0 as best fit.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>