drm/i915/selftests: Break out of the lrc layout test after register mismatch

AFter detecting a register mismatch between the protocontext and the
image generated by HW, immediately break out of the double loop.
Otherwise we end up with a second confusing error message.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210106123939.18435-1-chris@chris-wilson.co.uk