drm/i915: Convert intel_hdmi connector properties to atomic

intel_hdmi supports 3 properties, force_audio, broadcast rgb and
scaling mode. The last one is only created for eDP, so the is_eDP
in set_property is not required.

panel fitting and broadcast rgb are straightforward and only requires
changing compute_config.

force_audio is also used to force DVI mode, which means changes to
compute_config and mode_valid. mode_valid is called with
connection_mutex held, so it can safely dereference connector->state.

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/20170501133804.8116-11-maarten.lankhorst@linux.intel.com