drm/i915: Record ring->start address in error state

This is mostly useful for execlists where the rings switch between
contexts (and so checking that the ring's start register matches the
context is important).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/i915_drv.h
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gpu_error.c
Simple merge