[ozone/wayland] Fix hotspots for platform cursors on HiDPI displays.

Hotspots were wrong on outputs that have device scale factor greater
than 1.  That happened because the hotspot should be specified in DPI.

This CL fixes the math.

Bug: 1081784
Change-Id: Ie20b34df7d6fa35de3358303e2fafac3ff019977
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2726254


Auto-Submit: Alexander Dunaev <adunaev@igalia.com>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#858963}