drm/i915: More gt idling time with guc submission

On i915_selftest@live@gt_timelines, we create many contexts in loop and
create and submit request and then destoy contexts. Destroying the context
needs to disable scheduling, wait for G2H, deregister context and wait
for G2H to destroy each context. Idling of the gt has to wait for all
this to complete which is taking ~3sec for this test.

Hence we are increasing the igt_flush_test's timeout for gt idling to
3Sec.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128185209.18077-4-ramalingam.c@intel.com