drm/i915/gt: Force the GT reset on shutdown

Before we return control to the system, and letting it reuse all the
pages being accessed by HW, we must disable the HW. At the moment, we
dare not reset the GPU if it will clobber the display, but once we know
the display has been disabled, we can proceed with the reset as we
shutdown the module. We know the next user must reinitialise the HW for
their purpose.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/489


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200525151459.12083-1-chris@chris-wilson.co.uk