amdgpu: add libdrm as private requirement/dependency

Otherwise libdrm.so won't end up in the --libs, when one static links
libdrm_amdgpu.

Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>