drm/amdgpu: refine fault cache updates

Don't update the fault cache if status is 0.  In the multiple
fault case, subsequent faults will return a 0 status which is
useless for userspace and replaces the useful fault status, so
only update if status is non-0.

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