drm/i915/gt: Use a mmio read of the CSB in case of failure

If we find the GPU didn't update the CSB within 50us, we currently fail
and eventually reset the GPU. Lets report the value from the mmio space
as a last resort, it may just stave off an unnecessary GPU reset.

References: HSDES#22011327657
Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915134923.30088-4-chris@chris-wilson.co.uk