Actually that was the egl.pc missing which was failing the build
drm-tests always has a proper indirect dependency on libdrm

Package egl was not found in the pkg-config search path.
Perhaps you should add the directory containing `egl.pc'
to the PKG_CONFIG_PATH environment variable
No package 'egl' found
Package egl was not found in the pkg-config search path.
Perhaps you should add the directory containing `egl.pc'
to the PKG_CONFIG_PATH environment variable
No package 'egl' found
CC		null_platform_test.c ->  null_platform_test.pic.o
CC		null_platform_test.c ->  null_platform_test.pie.o
CC		vgem_test.c ->  vgem_test.pic.o
CC		vgem_test.c ->  vgem_test.pie.o
CC		vgem_fb_test.c ->  vgem_fb_test.pic.o
CC		vgem_fb_test.c ->  vgem_fb_test.pie.o
CC		swrast_test.c ->  swrast_test.pic.o
CC		swrast_test.c ->  swrast_test.pie.o
CC		atomictest.c ->  atomictest.pic.o
CC		atomictest.c ->  atomictest.pie.o
CC		bo.c ->  bo.pic.o
CC		dev.c ->  dev.pie.o
CC		bo.c ->  bo.pie.o
CC		modeset.c ->  modeset.pie.o
CC		nv12_test.c ->  nv12_test.pic.o
CC		gamma_test.c ->  gamma_test.pie.o
CC		dev.c ->  dev.pic.o
CC		gamma_test.c ->  gamma_test.pic.o
CC		modeset.c ->  modeset.pic.o
CC		bsdrm/example/stripe.c ->  bsdrm/example/stripe.pic.o
CC		bsdrm/example/stripe.c ->  bsdrm/example/stripe.pie.o
CC		nv12_test.c ->  nv12_test.pie.o
In file included from /build/smaug-cheets/tmp/portage/chromeos-base/drm-tests-9999/work/drm-tests-9999/modeset.c:5:
/build/smaug-cheets/usr/include/xf86drm.h:40:10: fatal error: 'drm.h' file not found
#include <drm.h>