drm/i915/hsw, bdw: Split power well set to enable/disable helpers

drm/i915/hsw, bdw: Split power well set to enable/disable helpers

We can reduce the code indentation by splitting the set helper to
separate enable/disable helpers. This also allows us to unify the
HSW/BDW and GEN9+ power well ops in follow-up patches, which introduces
some differences between the enable and disable helpers.

While at it also remove the redundant enable/disable debug messages,
the same info is printed already elsewhere.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1499352040-8819-12-git-send-email-imre.deak@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>