b2qt: heavy flickering on the beagleboneblack

Nearly every qt example with graphical output will have heavy flickering and/or other glitches when running on the beaglebone black.

The dmesg/journal will contain a lot of these:

[39771.715816] tilcdc 4830e000.lcdc: tilcdc_crtc_irq(0x00000020): FIFO underfow
[39771.931481] tilcdc 4830e000.lcdc: tilcdc_crtc_irq(0x00000020): FIFO underfow
[39772.375072] tilcdc 4830e000.lcdc: tilcdc_crtc_irq(0x00000020): FIFO underfow

This can be reproduced with any of the webengine examples (but it is not related to the webengine, can be seen with a lot of other examples too).