drm/selftests/mm: Insert cond_resched() between insert modes

kbuilder has begun running the selftests and reported a soft-lockup
inside __igt_insert(), so break up the test loop over different modes
with another call to cond_resched().

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107104131.5923-1-chris@chris-wilson.co.uk