drm/i915: set HDMI pixel clock in audio configuration

The HDMI audio expects HDMI pixel clock to be set in the audio
configuration. We've currently just set 0, using 25.2 / 1.001 kHz
frequency, which fails with some modes.

v2: Now with a commit message.

Reference: http://mid.gmane.org/CAGpEb3Ep1LRZETPxHGRfBDqr5Ts2tAc8gCukWwugUf1U5NYv1g@mail.gmail.com
Reference: http://mid.gmane.org/20130206213533.GA16367@hardeman.nu
Reported-by: David Härdeman <david@hardeman.nu>
Reported-by: Jasper Smet <josbeir@gmail.com>
Tested-by: Jasper Smet <josbeir@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>