drm/i915/skl: Fence registers on SKL are the same as SNB

v2: Rebased on top of the i915_gpu_error.c extraction.

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>