DanielP an orthogonal solution is to simply not use drmOpen. While it works,
sometimes, there's a lot of hidden gotchas.

Simply replace the pciaccess + drmOpen with drmDevice2 - see
libdrm/tests/drmdevice.c.

Notes:
* radeon-top does _not_ need to open the card node, hence no need for auth -
directly or via xcb
* using pciaccess, or drmDevice2 with DRM_DEVICE_GET_PCI_REVISION will wake up
your discrete GPU, even if you're looking for the stats of you APU - you want
to avoid that if possible.
* do not forget to close the fd - currently it's leaked.