[platforms/fbdev] Use a better way to correct activating framebuffer devices

Summary:
My change in 304528e8 has been reported to
cause issues for PostMarketOS https://gitlab.com/postmarketOS/pmaports/issues/204
I originally submitted the change to activate Framebuffer devices
as secondary framebuffer devices are not on by default, (such as a
secondary DisplayLink device). Changing attributes usually forced
some framebuffer devices to turn on, however I have found a more
accurate way to turn on framebuffer devices, without messing with
the color layout.

Reviewers: #kwin, davidedmundson, graesslin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19663