drm/i915: Dump the ringbuffer of the active request for debugging

Sometimes we need to see what instructions we emitted for a request to
try and gather a glimmer of insight into what the GPU is doing when it
stops responding.

v2: Move ring dumping into its own routine

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614122150.17552-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>