drm/i915/dp: Be smarter about connection sense for branch devices

If there's no downstream device, DPCD success is good enough.  If
there's a hotplug-capable downstream device, count the number of
connected sinks in DP_SINK_STATUS and return success if it's non-zero.
Otherwise, probe DDC and report appropriately.

v2: Check DP_SINK_STATUS instead of something unrelated to sink status.

Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>