drm/i915: Wait for concurrent global resets to complete

If we enter i915_handle_error() a second time and a global reset is
already in progress, we can simply wait for completion of the first
reset. Currently we exit early prior to the actual reset being
performed -- the worst of both worlds!

v2: Plug into the existing reset_queue, and remember that kselftests is
playing games with I915_RESET_BACKOFF to prevent hangcheck from screwing
up.
v3: Rename to i915_reset_device to fit in better with i915_reset_engine

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620095751.13127-2-chris@chris-wilson.co.uk