Please try the latest LibreELEC 9.0.1 / Rockchip Alpha 014 build (on a rockchip sbc with a "supported"/tested devicetree), it should work very good.
Do not stare blind at the alpha name, it is mostly there because there is features missing (HD-audio and full HDR infoframe metadata).
Rockchip is very much working and "supported" even if it is still flagged as alpha, the 9.0.0 / Alpha 013 and 9.0.1 / Alpha 014 images should work very well.
My focus for Rockchip at the moment is on mainline linux, hd-audio and adding more codecs to the rk v4l2 request api driver and ffmpeg hwaccel.
Even with the 4.9 aml kernel the drm/kms stack is unfortunately not used for video rendering. Compared to mainline (or RK 4.4) this mean that the new video rendering path in kodi (the direct-to-plane drmprime renderer) cannot be used. When amlcodec (and mmal) rendering is removed from kodi the kernel subsystems used for the display stack gets important.
The drm prime decoder + direct-to-plane renderer was first designed and tested for RK 4.4 kernel but thanks to using "modern" kernel subsystems it is working with any drm/kms platform that can render video on a dedicated video layer / plane and has been proven to work very good on mainline aml, rk, rpi, imx and other platforms.
Out of curiosity are you talking about the normal Libreelec github sources or is this something from your own github sources? i have built bables rockpro64 build and am interested in looking at others as well...
My normal toolchain is different then the Libreelec one and lately ive been fighting with it not finishing while CoreELEC's and balbes seem to work without any intervention on my part... Libreelecs probably just because of the huge coverage of devices its trying to handle and i never build on anything but real linux boxes... sofar i can get Coreelec and balbes to work on 15.04, 16,04 and 18.04 machines.
Sorry not that has anything really to do with my question, just thought i would qualify what i build with but am curious to have a look at what your referencing against and was not sure if it was your own github or libreelecs