drm/amdgpu: add debugfs interface for RAP test

After amdgpu driver loading successfully, we can use
RAP debugfs interface <debugfs_dir>/dri/xxx/rap_test
to trigger RAP test.

Currently only L0 validate test is supported.

v2: refine amdgpu_rap.h

Signed-off-by: Wenhui Sheng <Wenhui.Sheng@amd.com>
Reviewed-by: Guchun Chen <Guchun.Chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>