Making /usr/include/drm a symlink won't work as libdrm-dev still installs xgi_drm.h, via_3d_reg.h, mach64_drm.h, nouveau_drm.h and r300_reg.h there. You could make symlinks for individual files though. Beware that the versions you have might be too old for current intel mesa and 2d drivers (what I'm seeing on io.debian.net definitely is).
They are up-to-date. Hmm :-((( It leads me to questions. Is it good idea to provide some files in <drm/> from libdrm-dev and other one's directly from kernel headers ? Would be possible to serve all of them on non-linux architectures from libdrm-dev ? Sorry for these may be dumb questions, I do not know details of drm at all. Petr