>From which repository did you get your kernel? In case of a missing file I consult https://packages.debian.org/ "Search the contents of packages". There is also the package apt-file. The package linux-headers-5.4.0-4-common from bullseye contains a file /usr/src/linux-headers-5.4.0-4-common/include/drm/drmP.h
I don't have access to that GPU or experience with Radeon 520 Mobile. According to https://linux-hardware.org/index.php?id=pci%3A1002-6660-103c-8331 it should be supported by Linux kernel 5.4. Regards, Jörg.
