Upon gt resume, we first poison then sanitize the engine. However, our
testing shows that gen9 will very rarely retain the poisoned value from
the HWSP mappings of the execlists status registers. This suggests that
it is reading back from the HWSP, so rejig the register reset.

v2: Maybe RING_CONTEXT_STATUS_PTR is write masked. It is.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/1812Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200513100120.11617-1-chris@chris-wilson.co.uk