drm/amdgpu: return early if debugfs is not initialized

Check first if debugfs is initialized before creating
amdgpu debugfs files.

References: drm/amd#1686


Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>