drm/i915/gt: Discard stale context state from across idling

Before we idle, on parking, we switch to the kernel context such that we
have a scratch context loaded while the GPU idle, protecting any
precious user state. Be paranoid and assume that the idle state may have
been trashed, and reset the kernel_context image after idling.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200102131707.1463945-4-chris@chris-wilson.co.uk