[kiconloader] Set devicePixelRatio for pixmap before storing it in cache

When fetching a pixmap from the cache we set the desired devicePixelRatio, but we never set it when originally creating it.

This has two effects:

• In the uncached codepath we always return a QPixmap with dpr 1, only on subsequent calls for the same icon we set the correct dpr
• Every time we fetch a pixmap from the cache we change its dpr, causing a detach/copy

To avoid both set the dpr initially so that the cached pixmaps have the correct dpr set