[platforms/fbdev] Properly detect a BGR image format

Summary:
If the format of the framebuffer is BGR we cannot create an RGB image
format from it - the rendering is incorrect. Unfortunately QImage does
not support a BGR image format.

To solve this problem we still use an RGB image format but on rendering
the front buffer is rgbSwapped to convert the RGB image to a BGR image.

BUG: 365243
FIXED-IN: 5.7.2

Test Plan: Tested on a neon kvm

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2134