drm/amdgpu: Fix the KCQ hang when binding back

Just like the KIQ, KCQ need to clear the doorbell related regs as well
to avoid hangs when to load driver again after unloading.

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>