drm/amdgpu: add license to Makefiles

Was missing license text.

Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>