linuxfb: Make mouse cursor behavior match eglfs

On non-Android systems hideCursor should default to false, just
like it is done in eglfs. This is especially useful on udev-less
systems where currently one has to resort to setting
QT_QPA_FB_HIDECURSOR to "0" to enable the mouse cursor. This is
not ideal. Defaulting to showing the cursor unless disabled by
the environment variable or, in absence of that, the lack of a
mouse reported by udev is a better choice.

Change-Id: I7362ac47046179d5eb8ed8b44cf2c36c0fc23432
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>