Finally, omxplayer works fine on -current kernel without any tricks!!
According to this thread in official RPI repository in github,
https://github.com/raspberrypi/firmware/issues/763
when v3d, hdmi, pixelvalve, or hvs are enabled in FDT blog, firmware
turns off GPU functionality of VideoCore, expecting OS drivers control
it appropriately.
Therefore, these features are explicitly disabled in *.dtb, till we
add drivers for them (hdmi is already turned off in our DTS):
http://www.netbsd.org/~rin/rpi_dts_20180822.patch
With this patch, omxplayer works fine on Raspberry Pi 3B. And
everything including genfb and X also works fine as before.
OK to commit this fix?