drm/i915: cancel RPS work before disabling RPS

Ville noticed this while doing another review; we may as well cancel
this work just to make sure we don't try anything fancy after disabling
the RPS interfaces.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>