drm/i915: Haswell HDMI audio initialization

Added new haswell_write_eld() to initialize Haswell HDMI audio registers
to generate an unsolicited response to the audio controller driver to
indicate that the controller sequence should start.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Wang Xingchao <xingchao.wang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>