drm/i915: Handle force_audio correctly in intel_sdvo

Do the same as other connectors, attempt to detect hdmi audio in
the detect() callback, and only use the force_audio property as
override. Compute has_audio in pipe_config, and use that value
instead of the probed value directly.

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/20170501133804.8116-12-maarten.lankhorst@linux.intel.com