drm/amdgpu: Only clear dumb buffers if ring is enabled

The buffers should be cleared when possible but we also don't want
buffer creation to fail in the rare case where the ring isn't ready
during the call. This could happen during some suspend/resume sequences.

Cc: Christian König <ckoenig.leichtzumerken@gmail.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>