drm/amd: remove meaningless descending into amd/amdkfd/

Since commit 04d5e276

 ("drm/amdgpu: Merge amdkfd into amdgpu"),
drivers/gpu/drm/amd/amdkfd/Makefile does not contain any syntax that
is understood by the build system.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>