drm/i915/selftests: Disable capturing forced error states

When we are forcing the error for a selftest, we don't need to capture
the GPU state (typically).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200209230838.361154-1-chris@chris-wilson.co.uk