drm/i915: Skip capturing an error state if we already have one

drm/i915: Skip capturing an error state if we already have one

As we only ever keep the first error state around, we can avoid some
work that can be quite intrusive if we don't record the error the second
time around. This does move the race whereby the user could discard one
error state as the second is being captured, but that race exists in the
current code and we hope that recapturing error state is only done for
debugging.

Note that as we discard the error state for simulated errors, igt that
exercise error capture continue to function.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1467618513-4966-3-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>