Revert "drm/i915/guc: Disable global reset"

We have now prepared the guc reset paths to avoid taking struct_mutex, or
any other lock, and so it is now safe to re-enable.

References: fe62365f ("drm/i915/guc: Disable global reset")
Signed-off-by: Fernando Pacheco <fernando.pacheco@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419230015.18121-5-fernando.pacheco@intel.com