drm/i915: split power well 'set' handler to separate enable/disable/sync_hw

Split the 'set' power well handler into an 'enable', 'disable' and
'sync_hw' handler. This maps more conveniently to higher level
operations, for example it allows us to push the hsw package c8 handling
into the corresponding hsw/bdw enable/disable handlers and the hsw BIOS
hand-over setting into the hsw/bdw sync_hw handler.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Appease checkpatch's whitespace complaints.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>