drm/i915: Add plumbing for digital connector state, v3.

Some atomic properties are common between the various kinds of
connectors, for example a lot of them use panel fitting mode.
It makes sense to put a lot of it in a common place, so each
connector can use it while they're being converted.

Implement the properties required for the connectors:
- scaling mode property
- force audio property
- broadcast rgb
- aspect ratio

While at it, make clear that intel_digital_connector_atomic_get_property
is a hack that has to be removed when all connector properties
are converted to atomic.

Changes since v1:
- Scaling mode and aspect ratio are partly handled in core now.
Changes since v2:
- Split out the scaling mode / aspect ratio changes to a preparation
  patch.
- Use mode_changed for panel fitter, changes to this property
  are checked by fastset.
- Allowed_scaling_modes is removed, handled through core now.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170501133804.8116-6-maarten.lankhorst@linux.intel.com