drm/i915/vlv: enable HDA display audio for Valleyview2

This patch defines HD-Audio configuration registers and enables display audio
from HDA controller for Valleyview2.

v2: fix missing offset VLV_DISPLAY_BASE
v3: rename patch from 'enable HDMI audio' to 'enable HDA display audio', since
    it's for both HDMI and DP audio
v4: use enc_to_dig_port() to get port number, instead of using Haswell specific
    function intel_ddi_get_encoder_port()

Signed-off-by: Mengdong Lin <mengdong.lin@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>