drm/i915: Refactor to common helpers for prepare/finish between reset & wedge

Since both GPU reset and declaring the device wedged suspend ongoing
driver activity around a hard reset, we can reuse the same code to
reduce the likelihood of forgetting details surrounding reset from
either path.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190314084432.3740-1-chris@chris-wilson.co.uk