drm/i915: Kick the rps worker when changing the boost frequency

The boost frequency is only applied from the RPS worker while someone is
waiting on a request and requested a boost. As such, when the user
wishes to change the frequency, we have to kick the worker in order to
re-evaluate whether to apply the boost frequency.

v2: Check num_waiters to decide if we should kick the worker to handle
boosting.

Fixes: 29ecd78d ("drm/i915: Define a separate variable and control for RPS waitboost frequency")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180308142648.4016-1-chris@chris-wilson.co.uk