drm/i915/selftests: Increasing the sleep time for live_rc6_manual

Sometimes gt_pm live_rc6_manual selftest fails due to no power being
measured for the rc6 disabled period. Therefore increasing the rc6 disable
period from 250ms to 1000ms to rule out such sporadic failure.

v3:
- More descriptive and improved commit message (Anshuman)

Signed-off-by: Anirban Sk <sk.anirban@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240212050738.1162198-1-sk.anirban@intel.com