Replace dpi with width_mm/height_mm in qemu_edid_info.

Use it when set (non-zero) to compute the DPI and generate the EDID.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20200927145751.365446-3-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>