drm/i915: Flush the switch-to-kernel-context harder for DROP_IDLE

To complete the idle worker, we must complete 2 passes of wait-for-idle.
At the end of the first pass, we queue a switch-to-kernel-context and
may only idle after waiting for its completion. Speed up the flush_work
by doing the wait explicitly, which then allows us to remove the
unbounded loop trying to complete the flush_work in the next patch.

References: 79ffac85 ("drm/i915: Invert the GEM wakeref hierarchy")
Testcase: igt/gem_ppgtt/flind-and-close-vma-leak
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/20190507121108.18377-1-chris@chris-wilson.co.uk