drm/i915/bdw: on Broadwell, the panel fitter is on the pipe

So you can use the panel fitter while the power well is disabled and
you also don't need to set the "pipe" bit.

v2: Rebased on top of Jesse's pfit refactor, which moved pfit state
into the pipe_config.

v3: Rebase on top of the latest Haswell/panel fitter rework, which
neatly resolves a FIXME we have in this patch here:

v4: Rebase on top of the new power domain framework.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v1)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>