drm/amdgpu: fix NULL pointer in amdgpu_reset_get_desc

amdgpu_job_ring may return NULL, which causes kernel NULL
pointer error, using another way to print ring name instead
of ring->name.

Suggested-by: Lijo Lazar <Lijo.Lazar@amd.com>
Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>