drm/amdgpu: add cached GPU fault structure to vm struct

When we get a GPU page fault, cache the fault for later
analysis.

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>