drm/amdgpu: cache gpuvm fault information for gmc7+

Cache the current fault info in the vm struct.  This can be queried
by userspace later to help debug UMDs.

Cc: samuel.pitoiset@gmail.com
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>