drm/i915: Don't disable the GPU for older gen on wedging

If we issue a device level GPU reset on the older gen, it will disable
key components of the GMCH and the display engine. The purpose of
wedging is to simply prevent further GEM usage without disabling KMS, so
we need to be careful when we do issue the reset on wedging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180726085033.4044-3-chris@chris-wilson.co.uk


Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>