drm/amdgpu: Free CPER entry after committing to ring

Free CPER entry when it's committed to CPER ring to avoid memory leak.

Signed-off-by: Xiang Liu <xiang.liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>