gpu: ipu-v3: image-convert: fix image downsize coefficients and tiling calculation

This patch effectively reverts commit 912bbf7e ("gpu: ipu-v3:
image-convert: Fix image downsize coefficients") and replaces it with a
different solution based on the preceding patches.

The previous fix tried to solve the problem of intermediate tile size
between IC downsizing and main processing sections not being limited to
1024 pixels by downsizing the input image to a smaller intermediate size
in the downsizing box filter. This causes unnecessary blurring,
especially for scaling factors close to 1.

Now that the seam position calculation makes sure that the 1024 pixel
intermediate tile size limit is not exceeded, calculate the number of
tiles from the maximum of intermediate size and output size and avoid
unnecessary downsizing.

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