pmos-ui-plasma-mobile: enable software rendering

When rendering on framebuffer, always do software rendering. This should make it possible to boot up Plasma Mobile on most devices with downstream kernels, although terribly slow. Still better than a black screen though. Tested and working on the samsung-i9100 (see photo below).

We can improve the code and possibly make the rendering mode configurable per device once we experimented more with:

• llvmpipe vs. softpipe on various devices
• armv7 (around the corner in Alpine)
• a proper display manager like lightdm