drm/radeon: fix IB debugfs files for multiple cards

Storing pointers to the IBs in a static var just
leads to giving the same content back for all
cards in the system.

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