drm/i915/gt: Only delay the context barrier pm

It is strictly sufficient to only delay the intel_engine_pm_put from the
context barrier (and not from the context exit) in order to prevent the
gem_exec_nop contention. Adding the delay to the context exit incurs
noticably extra penalty for soft-rc6.

Fixes: edee52c9

 ("drm/i915/gt: Delay release of engine-pm after last retirement")
Testcase: igt/i915_pm_rc6_residency/rc6-idle
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323192029.20723-1-chris@chris-wilson.co.uk