drm/i915: Implement WaEnableHDMI8bpcBefore12bpc:snb, ivb

CPT/PPT require a specific procedure for enabling 12bpc HDMI. Implement
it, and to keep things neat pull the code into a function.

v2: Rebased due to crtc->config changes
    s/HDMI_GC/HDMIUNIT_GC/ to match spec better
    Factor out intel_enable_hdmi_audio()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Reviewed-By: Chandra Konduru <Chandra.konduru@intel.com>
Testecase: igt/kms_render/*
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>