drm/i915: Convert intel_sdvo connector properties to atomic.

SDVO was the last connector that's still using the legacy paths
for properties, and this is with a reason!

This connector implements a lot of properties dynamically,
and some of them shared with the digital connector state,
so sdvo_connector_state subclasses intel_digital_connector_state.

set_property had a lot of validation, but this is handled in the
drm core, so most of the validation can die off. The properties
are written right before enabling the connector, since there is no
good way to update the properties without crtc.

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/20170501133804.8116-13-maarten.lankhorst@linux.intel.com