Remove local hack in intel_atomic_commit_fence_wait().

Remove local hack in intel_atomic_commit_fence_wait().
This functions registers the proc on two wait_queues so either one
can wakeup the proc. Until lately this was not possible because a
mutex was held when setting up the wait state. Since the rework of
the sleep API and the cleanup of the linux wait.h implementation this
is no longer the case.
Tested by various people with different inteldrm versions.
OK kettenis@ jsg@