HDA036 spec states:
  DP (Display Port) indicates whether the Pin Complex Widget supports
  connection to a Display Port sink.  Supported if set to 1. Note that
  it is possible for the pin widget to support more than one digital
  display connection type, e.g. HDMI and DP bit are both set to 1.

Also export the DP pin cap in procfs.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>