drm/i915/gt: Ignore stale context state upon resume

We leave the kernel_context on the HW as we suspend (and while idle).
There is no guarantee that is complete in memory, so we try to inhibit
restoration from the kernel_context. Reinforce the inhibition by
scrubbing the context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200102131707.1463945-3-chris@chris-wilson.co.uk