drm/amdgpu: stop using sratch_reg in IB test

drm/amdgpu: stop using sratch_reg in IB test

scratch_reg0 is used by RLCG for register access usage
in SRIOV case.

both CP firmware and driver can invoke RLCG to do
certain register access (through scratch_reg0/1/2/3)
but rlcg now dosen't have race concern so if two
clients are in parallel doing the RLCG reg access
then we are colliding,

GFX IB test is a runtime work, so it is forbidden
to use scrach_reg0/1/2/3 during IB test period

note:
Although we can only have this change for SRIOV, but
looks it doesn't worth the effort to differentiate
bare-metal with SRIOV on the GFX ib test

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>