drm/i915: Record the position of the request upon error

So that we can tally the request against the command sequence in the
ringbuffer, or merely jump to the interesting locations.

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_debugfs.c
Simple merge

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

diff --cc drivers/gpu/drm/i915/i915_irq.c
Simple merge