eglfs: Round value returned from pixelDensity()

Avoid returning fractional scale factors, which we
do not support. This matches the pixelDensity()
implementation for the Xcb and Windows platform plugins.

Change-Id: I79156e802a0a436b9344f12d35d1f4861e20e7fa
Reviewed-by: Mitch Curtis <mitch.curtis@theqtcompany.com>