drm/radeon: fix debugfs handling v3

Having registered debugfs files globally causes
the files to not show up on the second, third
etc.. card in the system.

v2: fix crash on module unloading
v3: fix space indentation

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>