drm/i915: add noop power well handlers instead of NULL checking them

drm/i915: add noop power well handlers instead of NULL checking them

Reading code free of special cases wins over the small overhead of
calling a noop handler. Suggested by Jesse.

Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Jesse Barnes <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>