drm/i915: Wait for the struct_mutex on idling

When the system is idling, contention for struct_mutex should be low and
so we will be more efficient to wait for a contended mutex than
reschedule.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190430094405.6127-1-chris@chris-wilson.co.uk