drm/i915: Reset tasklet back to execlists after disabling guc

drm/i915: Reset tasklet back to execlists after disabling guc

When switching back to execlists, we also now need to restore the
tasklet handler.

Reported-by: Oscar Mateo <oscar.mateo@intel.com>
Fixes: 31de73501ac9 ("drm/i915/scheduler: emulate a scheduler for guc")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170318102859.24101-1-chris@chris-wilson.co.uk
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>