drm/i915: Skip switch-to-kernel-context on suspend when wedged

If the HW is already wedged, attempting to submit a request will
generate an -EIO. If we tried this during suspend, we would abort
whereas all we want to do is to go sleep and throw away the corrupt
state.

Fixes: 5ab57c70 ("drm/i915: Flush logical context image out to memory upon suspend")
Testcase: igt/gem_eio/suspend
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171130102951.14965-1-chris@chris-wilson.co.uk