drm/i915/selftests: Tweak the tolerance for clock ticks to 12.5%

Give a small bump for our tolerance on comparing the expected vs
measured clock ticks/time from 10% to 12.5% to accommodate a bad result
on Sandybridge that was off by 10.3%. Hopefully, that is the worst we
will see.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1802
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/20200428114307.5153-1-chris@chris-wilson.co.uk